C-\ H1orbl\ CC ll\33:12( IOCM7201| DUMP TRUCK The present invention relates to a dump truck having a deck that is provided on a vehicle body frame and capable 5 of moving and tilting with respect to the vehicle body frame. When a dump truck has a deck provided with a lower side openable flap on a vehicle body frame and capable of 10 moving and tilting with respect to the vehicle body frame, the lower portion of the flap is naturally open as the deck tilts with respect to the vehicle body frame. Therefore, unloading of a load from the deck of the dump truck is convenient. However, the lower side openable flap has 15 fulcrums at its upper portion. Even when the flap is open, the upper portion of the flap remains above the surface of the deck. For example, in order to place loads on the deck of the truck or unload loads from the deck of the truck, the loads that cannot pass through a space between the surface 20 of the deck and the flap need to be lifted to a position higher than the flap. When the flap is open, some of loads cannot pass through the space between the flap and the surface of the deck. In addition, some of the loads are long and therefore those loads cannot be unloaded. 25 A deck has a lower side openable flap that is 1 C\NH Ib C l\ : I : )0C /7/I'I I provided on the rear side of the deck and composed of a lower side openable frame and a window that closes an internal space of the lower side openable frame (refer to Patent Document 1). The lower side openable frame has a 5 lower side openable structure, and the window has a laterally openable structure. In the conventional technique, two first lock members and two second lock members are provided. The first lock members are adapted to fix the lower side openable frame to the deck, and the 10 second lock members are adapted to fix the window to the lower side openable frame. In this case, when only the second lock members are in a locked state during unloading of a load, the lower side openable frame and the window can be integrated with each other and used as a typical lower 15 side openable flap. When only the first lock members are in a locked state, and when the window is laterally open and retracted from the flap, a space located on the rear side of the deck can be widely open. This improves convenience of loading and unloading a load on and from the deck, and 20 significantly reduces the limitation of the size and shape of the load that can be unloaded from the dump truck. Patent Document 1: JP-A-08-11617 In the conventional technique, however, an operator 25 needs to manually handle the first or second lock members in 2 C\NR.iI..rth\DCC\ I.\: 3326_IDOC-4/7/2011 order to unload a load by opening a lower portion of the flap or laterally opening the window from a normal state in which both first and second lock members are in the locked state. In order to unload a load from the dump truck, open 5 the window and place another load on the deck and travel, it is necessary to manually unlock the first lock members, manually lock the second lock members, and manually lock the first lock members. The locking and the unlocking are cumbersome. 10 (1) A dump truck according to the present invention comprises: a cab; a vehicle body frame connected to a rear portion of the cab; a deck that is provided on the vehicle body frame and capable of moving and tilting; and a lower side openable flap constituting a side wall of the deck, 15 wherein the flap has: an outer frame that is pivotably attached to the deck to ensure that a lower portion of the flap pivots around an upper portion of the flap, the upper portion serving as a fulcrum of the pivoting movement; a gate that closes an internal space of the outer frame and is 20 pivotably attached to a side column of the outer frame to ensure that the gate horizontally pivots when the deck is in a non-elevated state; an automatic release lock mechanism that is latched to a lower portion of the outer frame and locks a movement of the lower portion of the flap to prevent 25 the lower portion of the flap from being open when the deck 3 C \NHiribl\,M CC\II.\:5 4:12;- 1lDOC-4)720I is in the non-elevated state, and that automatically unlocks the outer frame and allows the lower portion of the flap to be open when the deck moves and tilts; and a switching lock mechanism having: a lock member that is provided on the gate 5 and adapted to lock the gate; a lock member that is provided on the deck and adapted to lock the outer frame; a support member that is provided on the outer frame; and a lever handle that is held by the support member and capable of being selectively latched to the lock member adapted to lock 10 the gate or the lock member adapted to lock the outer frame. (2) A dump truck according to the present invention comprises: a cab; a vehicle body frame connected to a rear portion of the cab; a deck that is provided on the vehicle body frame and capable of moving and tilting; and a lower 15 side openable flap constituting a side wall of the deck, wherein the flap has: an outer frame that is pivotably attached to the deck to ensure that a lower portion of the flap pivots around an upper portion of the flap, the upper portion serving as a fulcrum of the pivoting movement; a 20 gate that closes an internal space of the outer 4 frame and is pivotably attached to a side column of the outer frame to ensure that the gate horizontally pivots when the deck is in a non-elevated state; an automatic release lock mechanism that is latched to a lower portion of the outer frame and locks a movement of the lower portion of the flap to prevent the lower portion of the flap from being open when the deck is in the non-elevated state, and that automatically unlocks the outer frame and allows the lower portion of the flap to be open when the deck moves and tilts; and a switching lock mechanism including: a first lock member that is provided on an outer wall surface of a side portion of the gate and adapted to lock the gate; a second lock member that is provided on an outer wall surface of a lower end portion of the gate, protrudes to a lower end portion of a cross beam of the outer frame and adapted to lock the gate; a third lock member that is provided on the deck and adapted to lock the outer frame; a support member that faces the second lock member and is provided on the cross beam of the outer frame; and a lever handle having a first latch portion that is rotatably held by the support member and slidable along the cross beam of the outer frame and capable of being selectively latched to the first lock member or the third lock member, and a second latch portion capable of being latched to the second lock member, and that allows the lever handle to be selectively set to any of two lock positions: a first lock position at which the first and second latch portions are latched to the first and second lock members, respectively, and the gate is fixed to the 5 outer frame at two locations; and a second lock position at which the second latch portion is in a non-latched state, the first latch portion is latched to the third lock member, the outer frame is fixed to the deck, and the gate is open. 5 (3) A dump truck according to the present invention comprises: a cab; a vehicle body frame connected to a rear portion of the cab; a deck that is provided on the vehicle body frame and capable of moving and tilting; and a lower side openable flap constituting a side wall of the deck, wherein the 10 flap has: an outer frame that is pivotably attached to the deck to ensure that a lower portion of the flap pivots around an upper portion of the flap, the upper portion serving as a fulcrum of the pivoting movement; a gate that closes an internal space of the outer frame and is pivotably attached to a side column of the 15 outer frame to ensure that the gate horizontally pivots when the deck is in a non-elevated state; an automatic release lock mechanism that is latched to a lower portion of the outer frame and locks a movement of the lower portion of the flap to prevent the lower portion of the flap from being open when the deck is in 20 the non-elevated state, and that automatically unlocks the outer frame and allows the lower portion of the flap to be open when the deck moves and tilts; a first lock mechanism having a lock member, a support member and lever handle, the lock member being provided on the outer frame and adapted to lock the gate, the 25 support member being provided on the gate, the lever 6 handle being held by the support member and capable of being latched to the lock member; and a second lock mechanism having a first bracket, a second bracket and a pin, the first bracket being provided on the outer frame and adapted to lock the outer frame, the second bracket facing the first bracket and being provided on the deck and adapted to lock the outer frame, the pin being inserted in the first and second brackets and adapted to fix the first and second brackets, wherein when the deck is in the non-elevated state, and locking of the first lock mechanism is released, the gate is capable of being open; in the state in which the locking of the first lock mechanism is released and the second lock mechanism locks the outer frame, even when the deck moves and tilts and locking of the automatic release lock mechanism is automatically released, the deck is capable of moving and tilting under the condition that the outer frame is maintained in a locked state and only the gate is open; and when the first lock mechanism locks the gate, the second lock mechanism unlocks the outer frame, the lower portion of the flap is capable of being open during the tilting movement of the deck under the condition that the outer frame and the gate are integrated with each other. (4) The dump truck described in any of items (1) to (3) preferably comprises a support member that is provided on the deck and adapted to hold the opening gate. (5) The dump truck described in item (3) is preferably configured to ensure that the first bracket has a first pin insertion hole and a second pin insertion hole, 7 the first pin insertion hole facing a pin insertion hole provided on the side of the second bracket, the second pin insertion hole being adapted to transiently place the pin. (6) The dump truck described in any of items (1) to (3) 5 preferably comprises a spacer that is provided under the gate and in contact with an upper surface of the cross beam constituting a bottom portion of the outer frame. (7) The dump truck described in any of items (1) to (3) preferably comprises a plurality of hinges that are adapted to 10 pivotably couple the gate with the outer frame and vertically arranged without a gap between the hinges. It is thus possible to highly maintain the convenience of unloading a load through the lower side openable flap, and improve the convenience of loading and unloading of a load. 15 Furthermore, it is possible to improve workability in the locking and unlocking of the flap. The present invention will now be described, by way of non-limiting example only, with reference to the accompanying 20 drawings, which are briefly described as follows: Fig. 1 is a side view of the entire configuration of a dump truck according to a first embodiment of the present invention. Fig. 2 is an enlarged view of the periphery of a flap 25 provided for the dump truck according to the first embodiment when viewed from the rear side of the dump truck. Fig. 3 is an enlarged horizontal cross sectional view of the periphery of the flap provided for the dump truck according to the first embodiment, taken along line III-III 8 of Fig. 2. Fig. 4 is an enlarged view of the periphery of the flap provided for the dump truck according to the first embodiment when viewed from the right side of the dump truck. Fig. 5 is an enlarged view of the periphery of the flap provided for the dump truck according to the first embodiment when viewed from the left side of the dump truck. Fig. 6 is a diagram showing an example of another configuration of the flap provided for the dump truck according to the first embodiment when viewed from the rear side of the dump truck. Fig. 7 is a diagram showing the example of the other configuration of the flap provided for the dump truck according to the first embodiment when viewed from the left side of the dump truck. Fig. 8 is an enlarged view of a switching lock mechanism provided for the dump truck according to the first embodiment when viewed from the rear side of the dump truck. Fig. 9 is a cross sectional view taken along line IX IX of Fig. 8. Fig. 10 is an enlarged view of the periphery of a flap provided for a dump truck according to a second embodiment of the present invention when viewed from the rear side of the dump truck. Fig. 11 is an enlarged horizontal cross sectional view of the periphery of the flap provided for the dump truck according to the second embodiment, taken along line XI-XI of Fig. 10. 9 Fig. 12 is an enlarged view of the periphery of the flap provided for the dump truck according to the second embodiment when viewed from the right side of the dump truck. Fig. 13 is an enlarged view of the periphery of the flap provided for the dump truck according to the second embodiment when viewed from the left side of the dump truck. Description of Reference Numerals 1 Cab 2 Vehicle body frame 10 Deck 12 Flap 15 Outer frame 15a Cross beam 15b, c Side column 16 Gate 25, 26 Hinge 30 Automatic release lock mechanism 40 Switching lock mechanism 41 First lock member 42 Second lock member 43 Third lock member 44 Support member 45 Lever handle 45a First latch portion 45b Second latch portion 51 Support member 55 Spacer 60 First lock mechanism 10 C-\NH IblI\DCC:\l.\58:12G_. DOCl//2011 61 Lock member 62, 63 Support member 64 Lever handle 70 Second lock mechanism 5 71 First bracket 72 Second bracket 73 Pin 74 Pin insertion hole 75 First insertion hole 10 76 Second insertion hole Embodiments of the present invention will be described below with reference to the accompanying drawings. Fig. 1 is a side view of the entire configuration of a dump 15 truck according to a first embodiment of the present invention. In Fig. 1, the left side of the drawing is regarded as the front side of the dump truck; the right side of the drawing is regarded as the rear side of the dump truck; the front side of the drawing is regarded as the left side of the dump truck; and the back side of the drawing is 20 regarded as the right side of the dump truck. As shown in Fig. 1, the dump truck according to the present invention has a vehicle 3 and a deck 10. The vehicle 3 has a cab 1 and a vehicle body frame 2. The vehicle body frame 2 is connected to a rear portion of the cab 1. The deck 10 is provided on the vehicle 25 body frame 2 and capable of tilting with respect to the vehicle body 11 frame 2. The vehicle body frame 2 is composed of frames 4 (only one frame 4 present on one side of the dump truck is shown in Fig. 1) and a cross member (not shown). The frames 4 are located on the left and right sides of the dump truck and extend in the direction connecting the front side of the dump truck and the rear side of the dump truck. The cross member connects the left and right frames 4 with each other at a predetermined position. Hinges 5 are provided at a left rear end portion and right rear end portion of the vehicle body frame 2, i.e., at rear end portions of the left and right frames 4. The hinges 5 serve as the fulcrum of the tilting movement of the deck 10 with respect to the vehicle body frame 2 and are located on the rear side of the deck 10. Deck frames 11 (only one deck frame 11 present on one side of the dump truck is shown in Fig. 1) are provided on a bottom surface of the deck 10. The deck frames 11 are located on the left and right side end portions of the dump truck and extend in the direction connecting the front side of the dump truck and the rear side of the dump truck. The deck frames 11 and the frames 4 of the vehicle body frame 2 cooperate with each other through a hydraulic cylinder (not shown) extensible and retractable by means of a hydraulic fluid supplied from a hydraulic pump (not shown) mounted on the vehicle 3. One end of the hydraulic cylinder is held by the deck frame 11 through a hoist mechanism (not shown), while the other end of the hydraulic cylinder is held by the frame 4 of the vehicle body frame 2. The hydraulic cylinder 12 is pivotably held by the deck frame 11 and the frame 4 of the vehicle body frame 2. In this case, a hydraulic fluid is supplied to the hydraulic cylinder to ensure that the hoist mechanism extends, and the hydraulic cylinder thereby extends. Then, the deck 10 pivots around the hinges S with respect to the vehicle body frame 2, stands up and becomes in a state indicated by an alternate long and two short dashes line of Fig. 1. After that, the hydraulic fluid is discharged from the hydraulic cylinder. Thus, the hoist mechanism becomes bent, and the hydraulic cylinder shrinks. The deck 10 then pivots around the hinges 5 with respect to the vehicle body frame 2. The deck 10 then returns to the state indicated by a solid line of Fig. 1, in which the deck 10 is in a non-elevated state. The deck 10 is composed of a bottom surface and side walls present on the front, rear, left and right sides. In the present embodiment, among the four side walls, the rear side wall constitutes a flap (tale gate) 12. The other three side walls constitute fixed walls. The left and right side walls may constitute flaps in place of the rear side wall. In addition, the left, right and rear side walls may constitute flaps, like a three way dump truck. In the present embodiment, the rear wall serves as the flap door as an example. However, the present invention can be applied to the case where the other side wall(s) serves as the flap(s). Fig. 2 is an enlarged view of the periphery of the flap 12 when viewed from the rear side of the dump truck. 13 Fig. 3 is a horizontal cross sectional view of the flap 12 taken along line III-III of Fig. 2. Fig. 4 is a diagram showing the flap 12 when viewed from the right side of the dump truck, and Fig. 5 is a diagram showing the flap 12 when viewed from the left side of the dump truck. As shown in Figs. 2 to 5, the flap 12 has a substantially U-shaped outer frame 15 and a gate 16. The outer frame 15 has a cross beam 15a and left and right side columns 15b and 15c. The cross beam 15a constitutes a bottom portion of the flap 12. The outer frame 15 is open upward. The gate 16 is adapted to close an internal space between the side columns 15b and 15c (of the outer frame 15). The outer frame 15 is pivotably attached to other portions (upper surfaces of rear end portions of the left and right side walls 19 and 20 in the present embodiment) of the deck 10 through hinges 17 and 18. The hinges 17 and 18 are attached to upper end portions of the left and right side columns 15b and 15c, respectively. The outer frame 15 pivots around upper portions (pins for the hinges 17 and 18) of the side columns 15b and 15c serving as the fulcrum. A lower portion of the outer frame 15 is open as the deck 10 tilts with respect to the vehicle body frame 2. The gate 16 is pivotably attached to the left side column 15b (or the right side column 15c) of the outer frame 15 through hinges 25 and 26 to ensure that the gate 16 substantially horizontally pivots when the deck 10 is in the non-elevated state. Each of the hinges 25 and 26 is composed of a pair of plates 28 attached to bosses 27. One 14 of the plates 28 of each of the hinges 25 and 26 is, for example, fixed to a left side surface (left side surface of the side column 15b) of the outer frame 15 under the condition that the bosses 27 protrude toward the rear side from a rear surface of the outer frame 15 when viewed from the left side of the outer frame 15 (refer to Fig. 5). The other of the plates 28 of each of the hinges 25 and 26 is, for example, fixed to a rear surface of the gate 16 under the condition that the bosses 27 protrude toward the left side from a left side end surface (left side surface of the side column 15b) of the outer frame 15 when viewed from the rear side of the outer frame 15 (refer to Fig. 2). That is, the one plate 28 constituting a part of the hinge 25 and the one plate 28 constituting a part of the hinge 26 are fixed to the left side surface of the side column 15b of the outer frame 16 and vertically arranged at an interval, while the other plate 28 constituting a part of the hinge 25 and the other plate 28 constituting a part of the hinge 26 are fixed to the rear surface of a left end portion of the gate 16 and vertically arranged at the same interval as that of the plates 28 attached to the left side surface of the side column 15b. Under the condition that the boss 27 of one of the plates 28 fixed to the outer frame 15 is located under the boss 27 of one of the plate 28 fixed to the gate 16, and the boss 27 of the other of the plates 28 fixed to the outer frame 15 is located under the boss 27 of the other of the plate 28 fixed to the gate 16, a single shaft 29 extends through the four bosses 27. The four plates 28 are coupled 15 with each other through the shaft 29. Fig. 6 is a diagram showing an example of another configuration of the flap 12 according to the present embodiment when viewed from the rear side of the dump truck. Fig. 7 is a diagram of the example shown in Fig. 6 when viewed from the left side of the dump truck. In the present embodiment, as shown in Figs. 2 to 5, a gap is present between the hinges 25 and 26 that are vertically arranged. A central portion of the shaft 29 is exposed. As shown in Figs. 6 and 7, the hinges 25 and 26 may be in contact with each other. In this case, there is no gap between the hinges 25 and 26 that are vertically arranged, and the hinges 25 and 26 cover the shaft 29. The boss 27 of the plate 28 (of the hinge 25) fixed to the outer frame 15 is contact with the boss 27 of the plate 28 (of the hinge 26) fixed to the gate 16. The length (measured in the axial direction of the shaft 29) of the boss 27 of each plate 28 is larger than that of the boss 27 of each plate 28 shown in Figs. 2 to 5. It should be noted that a small gap caused by a manufacturing error is be treated as the contact of the bosses 27. In the case where there is the gap between the hinges 25 and 26 as shown in Figs. 2 to 5, and if the pitch between the plates 28 fixed to the outer frame 15 does not accurately match the pitch between the plates 28 fixed to the gate 16, a gap between the bosses 27 of the plates 28 constituting the hinge 25 and/or a gap between the bosses 27 of the plates 28 constituting the hinge 26 may be present. 16 This may prevent the gate 16 from smoothly opening and closing. In this case, the weight of the gate 16 is substantially supported by the hinge 25 or 26 during the opening and closing. The lifetimes of the hinges 25 and 26 may be different from each other. As shown in the example of Figs. 6 and 7, the hinges 25 and 26 are vertically arranged and in contact with each other. In this configuration, the weight of the gate 16 is supported at a contact surface between the bosses 27 of the plates 28 constituting the hinge 25, a contact surface between the bosses 27 of the plates 28 constituting the hinge 26, and a contact surface between the boss 27 of the upper plate 28 of the hinge 26 and the boss 27 of the lower plate 28 of the hinge 25. The three gaps, which are the gap between the bosses 27 of the plates 28 constituting the hinge 25, the gap between the bosses 27 of the plates 28 constituting the hinge 26 and the gap between the boss 27 of the upper plate 28 of the hinge 26 and the boss 27 of the lower plate 28 of the hinge 25, are substantially removed. This can improve smoothness of the hinges 25 and 26. In the present embodiment, the two hinges 25 and 26 (four plates 28) are provided as an example. The number of the hinges (the number of the plates 28) may be changed. The deck 10 has an automatic release lock mechanism 30. The automatic release lock mechanism 30 locks a movement of the lower portion of the flap 12 to prevent the lower portion of the flap 12 from being open when the deck 10 is in the non-elevated state. The lock mechanism 30 17 automatically unlocks the flap 12 to allow the lower portion of the flap 12 to be open when the deck 10 moves and tilts with respect to the vehicle body frame 2. The automatic release lock mechanism 30 restrains the outer frame 15 under the condition that a hook 32 is engaged with a bracket 31 provided under the cross beam 15a of the outer frame 15 when the deck 10 is in the non-elevated state. When the deck 10 moves and tilts, the hook 32 is removed from the bracket 31 by a mechanism that moves in synchronization with the tilting movement of the deck 10. The removal of the hook 32 from the bracket 31 allows the lower portion of the outer frame 15 to be open. When the deck 10 moves and returns from a tilting state to the non-elevated state, the hook 32 is automatically engaged with the bracket 31, and the outer frame 15 is restrained by the deck 10. Detail configurations of the automatic release lock mechanism 30 are described in JP-A-58-49410, JP-A-59-25694, JP-A-60-21095 and the like. The outer frame 15 is fixed to the deck 10 mainly by the automatic release lock mechanism 30, while the gate 16 is fixed to the outer frame 15 by a switching lock mechanism 40. The switching lock mechanism 40 is adapted to fix the gate 16 to the outer frame 15. In addition, as described later, the switching lock mechanism 40 is adapted to fix the outer frame 15 to the deck 10 when the gate 16 is in an opening state. The switching lock mechanism 40 has a first lock member 41, a second lock member 42, three third lock members 18 43, a support member 44 and a lever handle 45. The first lock member 41 is adapted to lock the gate 16 and is provided on the rear surface (outer wall surface) of a right portion (or left portion) of the gate 16. The second lock member 42 is adapted to lock the gate 16 and is provided on the rear surface (outer wall surface) of a lower end portion of the gate 16. The second lock member 42 protrudes to a lower end portion (on the opposite side to the gate 16) of the cross beam 15a of the outer frame 15. The three third lock members 43 are adapted to lock the outer frame 15 and provided on a lower end portion of the right side wall 20 (or the left side wall 19) of the deck 10. The support member 44 faces the second lock member 42 and is provided on the cross beam 15a of the outer frame 15. The lever handle 45 is held by the support member 44. Each of the first to third lock members 41 to 43 has a hole 46 (only the hole 46 of the third lock member 43 is shown in Fig. 4). In the present embodiment, the holes 46 of the first and third lock members 41 and 42 extend through the first and second lock members 41 and 42 in the left right direction, respectively. The holes 46 of the third lock members 43 do not extend through the third lock members 43 and are open on the right side. The hole 46 of the first and the holes 46 of the third lock members 41 and 43 are arranged on the same circle having its center that is located at the hole 46 of the second lock member 42 when viewed from the right side of the dump truck as shown in Fig. 4. 19 Fig. 8 is an enlarged view of the switching lock mechanism 40 when viewed from the rear side of the dump truck. As shown in Fig. 8, the lever handle 45 has a first latch portion 45a and a second latch portion 45b. The first latch portion 45a is selectively inserted in any of the holes 46 of the first and third lock members 41 and 43 and can be latched to the any of the first and third lock members 41 and 43. The second latch portion 45b is inserted in the hole 46 of the second lock member 42 and can be latched to the second lock member 42. The first and second latch portions 45a and 45b extend in the left-right direction and are connected with each other via a handle portion 45c extending in a direction perpendicular to the left-right direction. In the present embodiment, the second latch portion 45b extending along the cross beam 15a of the outer frame 15 is longer than the first latch portion 45a. The length of the first latch portion 45a is slightly larger than the width of the right side column 15c of the outer frame 15. The second latch portion 45b is rotatably held by the support member 44. In addition, the second latch portion 45b is slidable along the cross beam 15a of the outer frame 15. The support member 44 is composed of brackets 44a and 44b that are arranged at a predetermined interval and on the cross beam 15a of the outer frame 15. The second latch portion 45b of the lever handle 45, which is inserted in the bracket 44a and 44b, has a stopper member 45d located 20 between the brackets 44a and 44b. The second latch portion 45b has a spring 47 located between the stopper member 45d and the right-side bracket 44b. The spring 47 urges the lever handle 45 to the side (left side) of the lock members 41 to 43. A part of the second latch portion 45b, which protrudes to the left side of the bracket 44a, is ensured to be longer than the maximum stroke of the lever handle 45. The maximum stroke is defined by the interval between the brackets 44a and 44b, the stopper member 45d, and the thickness of the spring 47 during the shrinkage of the spring 47. The switching lock mechanism 40 is structured to ensure that even when the lever handle 47 is pulled toward the right side against the spring 47, the second latch portion 45b is not removed from the support member 44. A bracket 48 is fixed to the right side column 15c of the outer frame 15 and faces the first lock member 41. The first latch portion 45a of the lever handle 45 is inserted in the first lock member 41 through the bracket 48. A grip 50 is provided on the outer wall surface of the gate 16 on the opposite side (right side in the present embodiment) to the hinges 25 and 26. The grip 50 is used to open the gate 16. As described above with reference to Fig. 1, a support member 51 for holding the opening gate 16 is provided on an outer wall surface of the left side wall 19 of the deck 10 (the support member 51 for holding the opening gate 16 is provided on an outer wall surface of the right side wall 20 of the deck 10 when the opening and 21 closing structure of the gate 16 is located on the opposite side to that in the present embodiment). When the gate 16 pivots 270 degrees from the state shown in Fig. 1, and contacts the outer wall surface of the left side wall 19, the support member 51 is inserted in the first lock member 41 provided on the gate 16 in a similar way to the insertion of the first latch portion 45a of the lever handle 45, and fixes the gate 16 located after the 270 degree pivoting to the left side wall 19. Fig. 9 is a cross sectional view taken along line IX IX of Fig. 8. As shown in Fig. 9, a spacer 55 is attached to a lower portion (bottom surface) of the gate 16. The spacer 55 may be made of metal and welded to the bottom surface of the gate 16. Alternatively, the spacer 55 may be made of resin and attached to the bottom surface of the gate 16 by means of a screw, an adhesive or the like. When the gate 16 is in a closed state, the spacer 55 is present between the gate 16 and the outer frame 15 and in contact with an upper surface of the cross beam 15a of the outer frame 15. It should be noted that a small gap caused by a manufacturing error can be treated as the contact of the spacer 55 with the upper surface of the cross beam 15a. The spacer 55 may be provided under the entire bottom surface of the gate 16. It is, however, preferable that the spacer 55 be provided under a part of the bottom surface of the gate 16 and at an appropriate location on the opposite side to the hinges 25 and 26 serving as the pivoting fulcrum in order to ensure 22 manufacturing accuracy and smoothness of the opening and closing of the gate 16. In the present embodiment, the spacer 55 is located in the vicinity of the second lock member 42 (refer to Fig. 8). The closing gate 16 is held by the hinges 25 and 26 on the fulcrum side and held by the spacer 55 on the opposite side to the fulcrum. Under the condition that the spacer 55 is not provided and the gate 16 is held by the hinges 25 and 26 (only one side of the gate 16 is held), when there is a gap between a lower portion of the gate 16 and the cross beam 15a of the outer frame 15, the gate 16 may shake due to shaking of the dump truck during a movement of the truck. In the present embodiment, the weight of the gate 16 is supported by the outer frame 15 at left and right appropriate locations. This can suppress the shaking of the gate 16 due to the shaking of the truck during the movement of the truck. In addition, loads applied to the hinges 25 and 26 and to the shaft 29 can be reduced. The spacer 55 serves to adjust the gap between the gate 16 and the outer frame 15. The manufacturing accuracy required for the gate 16 and the outer frame 15 can be reduced. According to the structure described above, for example, when the lever handle 45 is in the state (referred to as a first lock position) shown in Fig 8, and when the handle portion 45c of the lever handle 45 is pulled to the right side against an urging force of the spring 47, the first and second latch portions 45a and 45b of the lever handle 45 are pulled out of the first and second lock 23 members 41 and 42. The gate 16 is then unlocked. Then, under the condition that the lever handle 45 is pulled to ensure that an end portion of the first latch portion 45a is located on the right side of the outer frame 15, the lever handle 45 is rotated around the second latch portion 45b serving as the fulcrum. The first latch portion 45a is then inserted in the third lock member 43. The lever handle 45 becomes in a state (referred to as a second lock position) in which only the outer frame 15 is fixed to deck 10. When the lever handle 45 is located at the second lock position, the first latch portion 45a is latched to the third lock member 43 under the condition that the lever handle 45 is located on the right side of the first lock position. Even when the lever handle 45 is urged by the spring 47, the second latch portion 45b does not return to a location at which the second latch portion 45b is inserted in the second lock member 42, and the lever handle 45 is maintained in a non-latched state. When the lever handle 45 is located at the second lock position, the lever handle 45 restrains the outer frame 15 only and allows the gate 16 to be laterally open. When the gate 16 is open in this way, a rear portion of the deck 10 is widely open without an obstacle. The loading and unloading of a load on and from the deck 10 can be efficiently performed. Under the condition that the lever handle 45 is located at the second lock position, and when the gate 16 pivots and is held by the support member 51 and the deck 10 moves and tilts with respect to the vehicle body frame 2, 24 the locking of the automatic release lock mechanism 30 is released as the deck 10 moves and tilts with respect to the vehicle body frame 2. However, since the outer frame 15 is fixed to the deck 10 by the lever handle 45, a load placed on the deck 10 can be unloaded through an opening space (the internal space between the side columns 15b and 15c of the outer frame 15) of the gate 16. In this case, since the gate 16 is retracted from the rear portion of the deck 10 and there is no obstacle above the deck 10, an object having a larger height than that of the gate 16 and a long object can be easily unloaded from the deck 10. According to procedures opposite to those described above, the deck 10 returns to the non-elevated state, and the lever handle 45 returns to the first lock position. The first and second latch portions 45a and 45b are latched to the first and second lock members 41 and 42, respectively. The gate 16 is firmly fixed to the outer frame 15 at the two locations. Normally, this state is maintained. The automatic release lock mechanism 30 locks the flap to prevent the lower portion of the flap from being open. In this state, when the deck 10 moves and tilts with respect to the vehicle body frame 2 without handling of the switching lock mechanism 40, a load can be unloaded from the deck 10 in the same way as unloading of a load from a deck having a conventional lower side openable flap. When a load (earth and sand, and the like) placed on the deck can passes through a space between the flap 12 and the bottom surface of the deck 10, it is not necessary to handle the switching 25 lock mechanism 40. According to the present embodiment, the switching lock mechanism 40 is provided to allow the lever handle 45 to be selectively set to the two lock positions, i.e., the first lock position and the second lock position, while the automatic release lock mechanism 30 is provided. The position of the lever handle 45 is switched between the first and second lock positions to allow the gate 16 and the outer frame 15 to be integrated with each other and allow only the outer frame 15 to be restrained and the gate 16 to be open. Therefore, it is possible to highly maintain the convenience of unloading of a load through the lower side openable flap, and improve the convenience of the loading and unloading of a load. Furthermore, it is possible to improve workability in the locking and unlocking of the flap. In the case where work other than the loading and unloading is performed under the condition that the gate 16 is in the opening state, it is only necessary that the outer frame 15 be fixed to the deck 10 by the automatic release lock mechanism 30. When the lever handle 45 is set to the first lock position (in the normal state), and when the deck 10 moves and tilts with respect to the vehicle body frame 2, the movement of the lower portion of the flap 12 of the deck 10 is automatically unlocked by the automatic release lock mechanism 30 to allow the lower portion of the flap 12 to be open. It is not necessary to manually lock and unlock the outer frame 15 and the gate 16. In addition, the efficiency of loading and unloading a load on and from the deck is not 26 reduced. When the lever handle 45 is located at the first lock position, the first latch portion 45a of the lever handle 45 is fixed to the first lock member 41, and the second latch portion 45b serving as a rotational axis of the lever handle 45 is fixed to the second lock member 42. The outer frame 15 and the gate 16 can be firmly fixed to each other at the two locations through a single handling. In the present embodiment, when the lever handle 45 is located at the first lock position, not only the first latch portion 45a but also the second latch portion 45b are used to restrain the gate 16. It is, however, not necessary to limit the configuration of the lever handle 45 to this configuration as long as the handling of the lever handle 45 is ensured. It is not necessary to provide such a double lock structure. For example, the second latch portion 45b and the second lock member 42 used for the second latch portion 45b may be removed, and the first latch portion 45a may be selectively latched to any of the first and third lock members 41 and 43 under the condition that the strength of the first latch portion 45a is ensured. In the present embodiment, among the components of the switching lock mechanism 40, the first and second lock members 41 and 42 attached to the gate 16 are arranged on the inner side (left side) of a right end portion of the gate 16. If the first lock member 41 protrudes from the right end portion of the gate 16, and the gate 16 pivots, the portion of the first lock member 41 may interfere with 27 something, and may be a psychological burden for a worker from the perspective of safety. In the present embodiment, however, there is no structure protruding from the radius of the pivoting movement of the gate 16. This configuration is excellent from the perspective of safety and mental health. Fig. 10 is an enlarged view of the periphery of the flap 12 provided for the dump truck according to a second embodiment of the present invention when viewed from the rear side of the dump truck. Fig. 11 is a horizontal cross sectional view taken along line XI-XI of Fig. 10. Fig. 12 is a right side view of the periphery of the flap 12. Fig. 13 is a left side view of the periphery of the flap 12. In Figs. 10 to 13, the same parts as those described in the first embodiment or parts serving the same functions as those described in the first embodiment are denoted by the same reference numerals shown in Figs. 1 to 9 and their explanations are omitted. The configurations other than configurations described below are the same as those described in the first embodiment. The structure of a lock mechanism for the flap 12 according to the second embodiment is different from that of the lock mechanism for the flap 12 according to the first embodiment. In the second embodiment, the lock mechanism for the flap 12 includes a first lock mechanism 60, a second lock mechanism 70, and the automatic release lock mechanism 30 that is the same as that used in the first embodiment. The first lock mechanism 60 is adapted to fix the gate 16 to the outer frame 15. The second lock mechanism 70 is adapted 28 to fix the outer frame 15 to the deck 10. The first lock mechanism 60 has a lock member 61, support members 62 and 63, and a lever handle 64. The lock member 61 is provided on the right side column 15c of the outer frame 15 and adapted to lock the gate 16. The support members 62 and 63 face the lock member 61 and are provided on the gate 16. The lever handle 64 is held by the support members 62 and 63 and can be latched to the lock member 61. Each of the lock member 61 and the support members 62 and 63 has a hole 66 (only the hole 66 of the lock member 61 is shown in Fig. 12) that opens in the left-right direction. The lever handle 64 has a latch portion 64a. The latch portion 64a is inserted in the hole 66 of the lock member 61 and can be latched to the lock member 61. The latch portion 64a extends in the left-right direction. A handle portion 64b extends from a left end portion of the latch portion 64a and is perpendicular to the latch portion 64a. That is, the handle 64b is bent. The latch portion 64a is rotatably held by the support members 62 and 63 and slidable in the left-right direction. The support members 62 and 63 are provided on the gate 16 and arranged at a predetermined interval. The latch portion 64a of the lever handle 64, which is inserted in the support members 62 and 63, has a stopper member 64c. The stopper member 64c is located between the support members 62 and 63. The latch portion 64a has a spring 65 provided between the stopper member 64c and the left-side support member 63. The spring 65 urges the lever handle 64 to the side of the lock member 29 61 (right side). The support member 63 holding the handle portion 64b side of the lever handle 64 has a taper 63a that guides the handle portion 64b. In the present embodiment, the taper 63a has a shape that is upwardly inclined toward the left side. The handle portion 64b faces the downside in the normal state (shown in Fig. 10). When the handle portion 64b pivots upward, the handle portion 64b is guided by the taper 63a, and the entire lever handle 64 slides to the left side against an urging force of the spring 65. The latch portion 64a of the lever handle 64 is pulled out of the lock member 61. Then, the gate 16 is unlocked from the outer frame 15. When the handle portion 64b pivots 180 degrees and faces the upside, an end portion of the latch portion 64a of the lever handle 64 is inserted in the support member 62. The latch portion 64a is located on the left side of the right end portion of the gate 16 and in a retracted state. The second lock mechanism 70 has a first bracket 71, a second bracket 72 and a pin 73. The first bracket 71 is provided under the cross beam 15a of the outer frame 15 and adapted to lock the outer frame 15. The second bracket 72 faces the first bracket 71 and is provided on the side of the deck 10. The second bracket 72 is adapted to lock the outer frame 15. The pin 73 is inserted in the first and second brackets 71 and 72 and adapted to fix the brackets 71 and 72. The first bracket 71 has a first pin insertion hole 75 and a second pin insertion hole 76. The first pin 30 insertion hole 75 faces a pin insertion hole 74 located on the side of the second bracket 72. The second pin insertion hole 76 is used to transiently place the pin 73 (refer to Fig. 10) . One of end portions of the pin 73 has a diameter larger than the pin insertion holes 74 to 76. The other of the end portions of the pin 73 is bent around the pin serving as the fulcrum. The pin 73 inserted in the pin insertion holes is bent. That is, the other end portion of the pin 73 is inserted in the pin insertion holes and is bent. This shape prevents the pin 73 from being removed from the pin insertion holes. In the present embodiment, a support member for holding the gate 16 is provided on the left side wall 19 of the deck 10, although the support member is not shown in the drawings. In the first embodiment, the support member 51 is provided, which is designed to be inserted in the first lock member 41 that is adapted to lock the gate 16. In the present embodiment, a support member for pressing and holding the gate 16 may be provided. Alternatively, the same support member as the support member 51 may be provided, and a lock member adapted to receive the support member may be separately provided on the gate 16. In the present embodiment, the first lock mechanism 60 is in a locking state and the second lock mechanism 70 is in an unlocking state normally. In this state, when the deck 10 moves and tilts with respect to the vehicle body frame 2, the automatic release lock mechanism 30 automatically unlocks the flap 12. Then, the lower portion 31 of the flap 12 is open under the condition that the outer frame 15 and the gate 16 are integrated with each other. A load is unloaded from the deck 10. When the deck 10 returns to the non-elevated state, the automatic release lock mechanism 30 locks and restrains the flap 12 again. When the deck 10 is in the non-elevated state, and the locking of the first lock mechanism 60 is released, the gate 16 can be open. In this state, under the condition that the second lock mechanism 70 locks the outer frame 15, and the gate 16 is held by the support member provided on the left side wall 19 of the deck 10, even when the deck 10 moves and tilts, and the automatic release lock mechanism 30 automatically unlocks the flap 12, it is possible to cause the deck 10 to move and tilt under the condition that the outer frame 15 is locked and only the gate 16 is open. According to the present embodiment, since the automatic release lock mechanism 30, and the first and second lock mechanisms 60 and 70 are provided, it is only necessary to cause the deck 10 to move and tilt without performing the locking and unlocking in order to cause the lower portion of the flap 12 to be open and unload the load from the deck 10. In order to unload the load from the deck 10 under the condition that the gate 16 is open, it is necessary that the gate 16 be held by the support member and the outer frame 15 be locked by the second lock mechanism 70. In order to load and unload a load on and from the deck 10 under the condition that the deck 10 is in the non-elevated state and the gate 16 is open, it is only necessary to 32 release the locking of the first lock mechanism 60 from the normal state. In the present embodiment, it is possible to highly maintain the convenience of the unloading of a load through the flap having the lower side openable structure, and improve the convenience of the loading and unloading of a load on and from the deck. Furthermore, it is possible to improve the workability in the locking and unlocking of the flap. In the present embodiment, among the components of the first and second lock mechanisms 60 and 70, the support members 62 and 63, the lever handle 64, the first bracket 71 are attached to the gate 16. The latch portion 64a of the lever handle 64 protrudes from the right end portion of the gate 16 in the normal state. However, when the handle portion 64b is reversed and faces the upside in order to open the gate 16, the latch portion 64a is retracted and then located in the radius of the pivoting movement of the gate 16. Therefore, before the gate 16 pivots, all the components of the first and second lock mechanisms 60 and 70 are located on the inner side (left side) of the right end portion of the gate 16. As is the case with the first embodiment, this configuration is excellent from the perspective of safety and mental health. In the above description, the rear wall of the deck 10 serves as the flap 12 as an example. The present invention, however, can be applied to a dump truck having the side walls 19 and 20 serving as flaps and to a three way dump truck. When the side walls 19 and 20 serve as the 33 C\NRIrtI,\)C\Ill:V.M I3 I I )C4/7ri0 II flaps, in order to provide the support member, which is adapted to keep the gate fully open, on the deck, it is preferable that the laterally openable gate have a length equal to or smaller than the half of the overall length 5 (measured in the direction connecting the front and rear sides of the vehicle) of the side wall or equal to or smaller than the width of the rear wall (measured in the direction connecting the left and right sides of the vehicle). Many modifications will be apparent to those skilled in 10 the art without departing from the scope of the present invention. Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and 15 "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. The reference in this specification to any prior 20 publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the 25 field of endeavour to which this specification relates. 34