VEHICLE EXHAUST DEVICE FIELD OF THE INVENTION The present invention relates to an exhaust device having a pair of mufflers on the left and right sides of a vehicle body, in a vehicle such as a motorcycle on which a vehicle is mounted. Single cylinder engine. DESCRIPTION OF THE PRIOR ART Japanese Patent No. 2-241920 discloses an exhaust device with a pair of mufflers on the left and right sides of a vehicle body on a motorcycle on which a single-cylinder engine is mounted. This exhaust device includes an exhaust pipe extending from the single cylinder engine to one side of a rear portion of the vehicle body, a first muffle connected to a rear end portion of the exhaust pipe and supplied to one side of the vehicle. the body of the vehicle, a second muffler supplied to the other side of the vehicle body opposite the first muffle, and a connector pipe for connecting a front end portion of the second muffle and a front end portion of the first muffler to extend laterally to the body of the vehicle. vehicle. While it is required that the surface of an outwardly facing portion of the exhaust device be chromed, this chrome portion is possibly bleached by the exhaust heat. To prevent such discoloration, the exhaust pipe, connecting pipe and first and second mufflers must be formed to have a double pipe structure, and the outer surface of each component must be chromed. However, the connection between these structural components of double pipe is complicated, and requires a lot of time to manufacture. Also, if the connector portion is formed to have a single pipe structure, the entire chrome surface of the exhaust device becomes discontinuous and partial discoloration occurs, so that it is difficult to maintain the appearance of the exhaust pipe in good condition. . Accordingly, an object of the present invention is to provide an exhaust device for use with a single-cylinder engine, where a pair of mufflers can be supplied on opposite sides of a vehicle body without damaging its appearance, being able to maintain a good appearance with easy manufacturing. BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a top plan view illustrating the principle of the present invention. Figure 2 is a left side view showing the appearance of a vehicle body of a motorcycle to which the present invention is applied. Figure 3 is a right side view of the motorcycle. Figure 4 is a planar top view of the motorcycle. Figure 5 is a bottom plan view of the motorcycle. Figure 6 is a left side view of a left part of an exhaust device where a first cover was separated. Figure 7 is a top plan view of the left part. Figure 8 is a horizontal sectional view of the left part, where an exhaust pipe is not shown. Figure 9 is an enlarged cross-sectional view taken along line 9-9 of Figure 7. Figure 10 is a horizontal sectional view of a right part of the exhaust device. Figure 11 is a vertical sectional view of the right part. Figure 12 is a side view of a second cover. Figure 13 is a top plan view of the second cover. Figure 14 is an enlarged cross-section taken along line 14-14 in Figure 13. DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows the principle of the present invention. As shown in Figure 1, the vehicle scape device according to the present invention includes a single cylinder engine 1, an exhaust pipe 2 extending from the single cylinder engine 1 to one side of a rear portion of a vehicle body, a bypass member and gasket 3 connected to a rear end portion of the exhaust pipe 2, a first muffle 4 connected to a rear end portion of the bypass member and gasket 3, and supplied to one side of the vehicle body, a second muffle 5 supplied to the other side of the vehicle body opposite the first muffle 4, a connector tube 6 connecting a front end portion of the second muffle 5 and the branch and seal member 3, and extending laterally through the body of the vehicle, a first cover 7 for covering an external lateral surface of the branch member and seal 3, and a second cover 8 for covering an external lateral surface of a connector portion between a front portion of the second muffle 5 and the connector tube 6. Exhaust pipe 2, the first muffler 4 and the second muffle 5 have a double pipe structure with chromed exterior. The branch member and gasket 3 and the connector tube 6 have a single pipe structure. The first and second covers 7 and 8 are chromed. Preferably, a front end portion of the second cover 8 is curved toward the center of the vehicle body, and an imaginary line as a continuous forward extension from the front end portion of the second cover 8 is positioned before the connector tube axis . The rear end portion of the exhaust pipe 2 has a double pipe structure, and the front and rear portions of the first and second mufflers 4 and 5 both have a double pipe structure, connected together by the branch and seal member 3. and connector tube 6, where both have a single pipe structure. According to this, the connection structure of each connector component can be simplified, and the exhaust device can be easily manufactured. Also, the external side surfaces of these single pipe structural components 3 and 6 prone to discoloration because the heat of the exhaust are covered with the first and second covers 7 and 8, both chromed, ie concealed from a side view . The first and second covers 7 and 8 are of continuous appearance to the mufflers. According to this, the first and second covers 7 and 8 easily look like continuous porcines of the exhaust device, instead of simple separate covers that could be seen as being elevated from the surrounding parts. In this way, the appearance of the exhaust device can be made in such a condition that it is essentially chrome-plated uniformly and without discolorations due to the heat of the exhaust. Although the single-cylinder engine is mounted on the vehicle, the pair of mufflers 4 and 5 can be provided on opposite sides of the vehicle body without damaging the appearance, and it is possible to obtain an exhaust system with good appearance, and easy to use. manufacture. Also, in the event that the front end portion of the second cover 8 is curved toward the center of the vehicle body, and the imaginary line extends continuously forward from the front end portion of the second cover 8 it is positioned. before the axis of the connector pipe 6, the second muffle 5 appears as directly connected through the second cover 8 to a virtual exhaust pipe 9 which is shown in Figure 1. Furthermore, the very presence of the second cover 8 is not captures visually, thereby providing a natural feel to the exhaust and further enhances the appearance. A preferred embodiment of the present invention will now be described with reference to the drawings. Figure 2 is a left side view showing the appearance of a vehicle body to which the present invention applies; Figure 3 is a right elevated view of the motorcycle; Figure 4 is a top plan view of the motorcycle; Figure 5 is a planar bottom view of the motorcycle; Figure 6 is a left side view of a left part of an exhaust device where a first cover was separated; Figure 7 is a top plan view of the left part; Figure 8 is a horizontal sectional view of the left part, where an exhaust pipe is not shown; Figure 9 is an enlarged cross-sectional view taken along line 9-9 of Figure 7; Figure 10 is a horizontal sectional view of a right part of the exhaust device; Figure 11 is a vertical sectional view of the right side; Figure 12 is a side view of a second cover; Figure 13 is a top plan view of the second cover; Figure 14 is an enlarged cross-section taken along the line 14-14 in Figure 13. Referring to Figures 2 to 5, the motorcycle includes a single-cylinder engine 1 supported by a body support 12 between a front wheel 10 and a rear wheel 11. An exhaust pipe 2 connected at one of its ends to the engine 1 extends rearwardly from the underside of the engine 1 to the left side of the vehicle body. The other end of the exhaust pipe 2 is connected by a bypass member and gasket 3 to a first muffle 4 which extends on the left side of the rear tire 11. A second muffle 5 is supplied to the right side of the vehicle body to extend on the right side of the rear tire 11. A front end portion of the second muffle 5 is connected to the bypass member and gasket 3 by a connector tube 6 extending laterally to the vehicle body. An outer side surface of the bypass member and gasket 3 is covered with a first cover 7 to be externally invisible from the left side of the vehicle body. Similarly, an outer side surface of a connector portion between a front portion of the second muffle 5 and the connector tube 6 is covered with a second cover 8 to be externally invisible from the right side of the vehicle body. A front bifurcation 13 supporting the front wheel 10 is pivotally supported to a head 14, and is directed by the handle 15. The reference number 16 denotes a hand grip, and the reference number 17 denotes a steering lock. The front wheel includes a spoke wheel 18 formed of a light alloy. A disc brake 19 is provided on one side of the spoke wheel 18, and is braked by a balata 20. The reference numeral 21 denotes a front bumper. An upper portion of a cylinder portion 22 of the engine 1 is covered with a front side cover 23. A carburetor 24 is supplied after the cylinder portion 22. A self-starting engine 26 is provided in an upper portion of a crankcase 25. , located under the cylinder portion 22. A gearshift pedal 27 is provided in a lower front portion of the crankcase 25 to the left side of the vehicle body, and a brake pedal 28 is provided in the same portion on the side right of the body of the vehicle. The body frame 12 is of the cradle type, including a main frame 12a extending longitudinally of the vehicle body to an upper portion thereof, a pair of left and right front lower frames 12b extending downward and rearwardly. from the head 14, a pair of left and right lower frames 12c extending substantially horizontally backward from the lower ends of the left and right front lower frames 12b and supporting a lower portion of the crankcase 25, a pair of tie rods left and right 12d extending up and back from the rear ends of the lower left and right frames 12c, and a pair of lower left and right central frames 12e extending down and back from the rear end of main frame 12a. A pair of left and right foot supports 26 for a driver are mounted on the left and right lower frames 12c at their respective front portions. A lateral support 30 is supported by pivoting the left lower frame 12c to a position after the rest for left foot 29. A pair of plates with left and right pivot 31 are supported with the left and right rear braces 12d and the lower central frames left and right 12e in their respective lower extreme portions. Each pivot plate 31 is provided with a pivot portion 32 for pivotally supporting a forward end portion of a rear swing arm 33. A pair of left and right foot rest stops 34 for a passenger is mounted on the left and right rear braces 12d. A fuel tank 35 is supported on the main frame 12a on the cylinder portion 22. A driver's seat 36 and a passenger seat 37 are provided in tandem behind the fuel tank 35. The passenger seat 37 is supported on a rear fender body portion 38. A rear fender portion 39 extends rearwardly and downwardly from the rear end of the rear fender body portion 38. A pair of left and right fender rails 40 are provided on the surfaces left and right of the rear defense body portion 38 to extend rearwardly from the driver's seat 36. A vertically extending backrest 41 is supported on its left and right lower ends on the rear portions of the left handrail rails and right 40. A backrest 42 is supported on the back strap 41 in its upper portion. In Figures 2 and 3, the reference number 43 denotes an upper bridge, 44 a lower bridge, 45 a side cover and 46 an essentially disk-shaped wheel. As apparent in Figure 5, a transverse tube 47 is provided between the left and right lower frames 12c in the vicinity of a lower rear position of the crankcase 25. A pivot portion 29 of a main support 48 is supported by pivoting on the transverse tube 47. The pivot portion 49 is located before the connector tube 6, and the main holder 48 in its retracted condition is located under the connector tube 6. Accordingly, the connector tube 6 is covered on its front and bottom sides with the main support 48 during the running of the vehicle, thus being protected against some stone that jumps from the front wheel 10. The electric parts mounted on the motorcycle include a headlight 50, front directional signals 51, horn 52, rear directional signals 53 and rear light 54. As apparent in Figure 4, several indicators 55 are provided on the upper bridge 43. A projecting portion 56 e is formed on the upper surface of the fuel tank 35, and a meter 58 is provided in the projecting portion 56 in a position prior to a fuel intake 57. The structure of the exhaust system will be described in detail below, with reference to Figures 6 to 14. Although not shown, the exhaust pipe 2 has a double pipe structure and is generally chromed. A nut portion 60 is provided on the outer side surface of the exhaust pipe 2 at its rear end portion. As apparent in Figure 8, the branch and seal member 3 has a single tube structure, and is integrated by welding to the forward end of the first muffle 4. A clamp 51 is mounted on the outer lateral surface of the branch member and gasket 3. The clamp 51 is formed with two hooks 52 that rise from the outer side surface of the bypass member and gasket 3. The inner side surface of the bypass member and gasket 3 is formed by the side connection opening 63, and a left connector tube member 6a is connected to the side connection opening 63. The left connector tube member 6a has a connector end portion 64 adapted to be attached to a gasket 65 (see Figure 10) of a connector tube member right 6b. That is, the connector tube 6 can be divided into its intermediate portion in the right and left connector tube members 6a and 6b, and has a single pipe structure. The first muffle 4 has a double pipe structure composed of an outer wall 66 and an inner wall 67, and the internal structure of the first muffle 4 is similar to that of a known muffler, so that the internal space is separated into a plurality of cameras. The external surface of the external wall 66 is generally chromed. A transverse hole 68 is formed in a front end portion of the first cover 7. As is apparent in Figure 9, two tie rods 69 are mounted on the inner surface of the first cover 7 at its rear portion, to form two holes similar to tunnels 69a for joining respectively to the two hooks 52. As is apparent in Figure 10, the right connector tube member 6b is bent at its rear end portion to extend rearwardly, and is connected to a front end portion of the second hub 5 The second muffle 5 is similar in structure to the first muffle 4, that is, the second muffle has a double pipe structure composed of an outer wall 70 and an inner wall 71, and the interior space of the second muffle 5 is separated into a plurality of cameras. The external surface of the outer wall 70 is generally chromed. Only the front end portion of the second muffle 5 is formed as a front plug 72 with a single pipe structure. The front plug 72 is attached and welded to the front end of a body portion of the second muffle 5 having a double pipe structure. The rear end portion of the right connector tube member 6b is connected to the front plug 72. A clamp 73 with two hooks 74 is welded to the outer surface of the front plug 72. The hooks 74 are raised from the outer surface of the front plug 72. and they extend forward. As apparent in Figures 12 to 14, the second cover 8 has a structure essentially similar to that of the cover 7, and the outer surface of the second cover is also generally chromed. As apparent in Figure 13, the second cover 8 is curved in such a way as to form a curved line extending from the front cap 72 essentially conforming to the bent portion 75 of the right connector tube member 6b, and a front end portion 76 of the second cover 8 is directed towards the center of the vehicle body. That is, an imaginary line A that shows a continuous forward extension from the front end portion 76 of the second cover 8 is directed towards the engine 1 located at the central lateral position of the vehicle body (see also Figure 1). As is apparent from Figure 10, the imaginary line A forms an angle? with respect to the axis B of the right connector tube member 6b. The further the imaginary line A extends away from the forward end portion 76 of the second cover 8, the greater the distance between the imaginary line A and the axis B. The axis B of the right connector tube member 6b is identical to the axis of the connecting tube 6. A transverse hole 77 is formed in the front portion of the second cover 8. As shown in Figure 14, two tie rods 78 are mounted on the inner surface of the second cover 8 in its rear portion, to form two tunnel-like holes 79 to join respectively the two hooks 74. The structures of the first and second mufflers 4 and 5 will now be briefly described. The first muffle 4 is connected at its open front end of the bypass member and gasket 3., and the interior space of the first muffle 4 is separated in the second and third first expansion chambers 84, 85 and 96 by the first and second spacers 81 and 82 and a rear stopper 83. The first expansion chamber 84 and the third chamber of expansion 86 are communicated with each other by a first communication tube 87. The third expansion chamber 86 and the second expansion chamber 85 communicate with each other by a second communication tube 88. The second expansion chamber 85 and the atmosphere are they communicate with each other by a third communication tube 89. The bypass member and seal 3 is provided with a separator 80 located behind the side connection opening 63. As is apparent from an enlarged view in Figure 8, the separator 80 has a central hole 80a for passing the exhaust pipe 2 and a plurality of openings 80b formed around the central hole 80a. A part of the exhaust gas discharged from the exhaust pipe 2 into the first expansion chamber 84 is passed through the openings 80b of the separator 80, and flows into the side connection opening 63, as shown by the arrow P in Figure 8. Similarly, the interior space of the second muffle 5 is separated into first, second and third expansion chambers 94, 95 and 96 by first and second spacers 91 and 02, and a rear plug 93. The first expansion chamber 94 and the third expansion chamber 96 communicate with each other via a first communication tube 97. The third expansion chamber 96 and the second expansion chamber 95 communicate with each other by a second communication tube 98. The second expansion chamber 95 and the atmosphere are communicated with each other by a third communication tube 99. Next, the operation of this preferred embodiment will be described. When mounting the first cover 7, it is placed first on the surface of the outer side of the bypass member and joint 3, and then moved back to make a connection between the hooks 52 and the holes 69a of the braces 69 mounted on the inner surface of the first cover 7, as shown in Figures 6 and 7. Subsequently, the hole 68 formed in the front end of the first cover 7 is aligned with the nut portion 60 formed in the rear portion of the exhaust pipe 2, and a bolt (not shown) is joined with the nut portion 60 through the hole 68, thereby securing the first cover 7 with the exhaust pipe 2 and the bypass member and gasket 3. Therefore, the first cover 7 is mounted on the bypass member and gasket 3 to cover the surface of the outer side of the bypass member and gasket 3, thereby hiding it from the side view. Also, the first cover 7 has the appearance of continuing towards the rear end portion of the exhaust pipe 2 and the front end portion of the first muffle 4. When mounting the second cover 8, it is first placed on the outer side surface of the second cover. the bent portion 75 of the right connector tube member 6b, and then moves backwards to realize the attachment of the hooks 74 with the holes 79 of the struts 78, as shown in Figures 10 to 14. The hole 77 formed in FIG. the front end portion of the second cover 8 is aligned with a nut portion 75a (see Figure 10) formed in the bent portion 75, and a screw (not shown) is attached with the nut portion 75a through the hole 77, thereby securing the second cover 8 to the right connector tube member 6b and the front cap 72. According to the foregoing, the second cover 8 is mounted in such a way that it covers the surfaces of the outer side of the container. or less the bent portion 75 of the right connector tube member 6b and the front plug 72. Thus, a portion of these single tube structural components that appear out of the vehicle body is hidden from the side view. As a result, all the externally visible portions of the exhaust device on the left and right sides of the vehicle body appear as if they were essentially uniformly chromed. According to this, even if the single tube structural components (3, 6, 72) are discolored by the exhaust heat, the appearance of the vehicle body is not damaged. In addition, the first chrome cover 7 looks as if it continues to the exhaust pipe 2 and the first muffle 4, both chromed, and the second chrome cover 8 similarly looks as if it continues to the second chrome 5 muffle. According to this, the first and second covers 7 and 8 immediately look like continuous portions of the exhaust device, instead of separate covers that could look as if they were rising from the surrounding parts. Accordingly, the appearance of the exhaust device as a whole can be left in a condition such that it is essentially uniformly chrome-free without discolorations caused by the heat of the exhaust. Accordingly, the entire bypass member and seal 3, the connector tube 6 and the front plug 72 hidden by the first and second covers 7 and 8 can be formed as a single tube structural component without chroming. When connecting the rear end portion of the exhaust pipe 2, the first muffle 4 and the second muffle 5, where each has a double pipe structure through the single tube structural component above, the connecting structure of each connecting component it can be simplified, and the exhaust device can be easily manufactured. Although the single-cylinder engine 1 is mounted on the motorcycle, the pair of mufflers 4 and 5 can be provided on opposite sides of the vehicle body without causing damage to the appearance, and it is possible to supply an exhaust system with good appearance, and easy to manufacture In addition, the front end portion of the second cover 8 is curved in such a way that it can be directed towards the engine 1 located in the lateral central position of the vehicle body, and the imaginary line A extending continuously from the end portion. front of the second cover 8 is placed before the axis B of the connector tube 6 (see Figures 10 and 13). According to this, the second muffle 5 looks as if it is directly connected to a virtual exhaust pipe 9 (see Figure 1), which is connected to the engine 1. Furthermore, the second cover 8 looks as if it were part of the second muffle 5, and the very presence of the second cover 8 is not visually captured, thus providing a natural sensation to the exhaust device, and further improving the appearance.