JPS63266417A - Laser beam priner - Google Patents

Abstract

PURPOSE:To enable printing of sharp multiple multi-color images without having positional deviations by using a means for detecting the rotational moving amt. of a photosensitive drum to determine the timing in the sub-scanning direction of the laser beams of plural optical scanning systems. CONSTITUTION:An external apparatus 42 outputs a command for starting printing upon receipt of a ready signal from a printing control circuit 41. The photosensitive drum 21 is rotated in an arrow direction by this command and a clock sensor 2 outputs the moving amt. of the drum 22 as a clock signal from a disk 1 rotating at the same speed as the speed of the drum 21. The control part 41 transmits a request signal VSREQ for vertical synchronization in accordance with the first clock signal and the apparatus 42 returns a vertical synchronization signal VSYNC. Scanning of the 1st optical scanning system 18 is started by this signal and the 2nd optical scanning system is started similarly by the prescribed moving signal of he drum 21. The positional deviations of the 1st and 2nd scanning systems are eliminated and the printing of the sharp multiple multicolorimages is executed by constituting the printer in the above-mentioned manner.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a laser beam printer that forms multiple images using an electrophotographic process.

(Prior Art) Conventionally, this type of device has a plurality of optical scanning systems that scan a laser beam, and furthermore, the image carrier (recording medium) is scanned by the laser beam scanned by the optical scanning system (laser scanner). Laser beam printers have been developed that have a plurality of developing devices that develop electrostatic latent images formed on the surface using respective developing colors, and are capable of obtaining multicolor multiple images in one electrophotographic process step.

FIG. 4 is a cross-sectional view illustrating the configuration of a multiplex printing type laser beam printer. Reference numeral 11 indicates the printer body, and reference numeral 12 indicates a paper feed cassette, which accommodates the transfer sheet P, and is driven by the pickup roller 13. Sent from 12. A feeding roller 14 conveys the transfer sheet P sent out from the paper feeding cassette 12 via a conveying guide 15 to a position where the registration rollers 17 are arranged. 16 is a pre-registration sensor, and this pre-registration sensor 16 is connected to the feeding roller 14.
After a predetermined period of time has elapsed since the transferred sheet P was detected, the driving of the feeding roller 14 is stopped. 18 is the first
With the optical scanning system, the scanner motor 18a has a single rotating polygon mirror (
polygon mirror) 18b, semiconductor laser 18C1 imaging lens 18d, folding mirror 18e1 reflecting mirror 18f
, a beam detector 18g, etc., and rotates a laser beam LBI at a constant speed by a scanner motor 18a, which is modulated based on a first image signal VDO1 sent from an external device 42, which will be described later.
is deflected by the imaging lens 18d and the folding mirror 18.
e, the photosensitive drum 21 rotating at a predetermined speed is horizontally scanned. The reflective mirror 18f is a polygon mirror 18b.
The laser beam LBI is provided near the end of the scanning area in the central axis direction of the drum, and receives a laser beam LBI deflected and scanned by a polygon mirror 18b immediately before image writing, and guides the received laser beam LBI to a beam detector 18g. The beam detector 18g detects the laser beam LBI reflected by the reflection mirror 18f, and sends a horizontal synchronization signal BDI for determining the image writing timing in the main scanning direction of the photosensitive drum 21 to an external device 42, which will be described later, and serves as an image data supply source. A first image signal VDO1 is output to the printer control unit 41 for forming an electrostatic latent image that is developed in a first developing color (black) in synchronization with the first horizontal synchronization signal BDI. , a laser beam LBI modulated based on this first image signal VDO1 is emitted from the semiconductor laser 18c. 19 is a second optical scanning system, which is a scanner motor 1;
9a1 rotating polygon mirror (polygon mirror) 19b, semiconductor laser 19C2 imaging lens 19d, folding mirror 19e
.

A laser beam LB2 is composed of a reflecting mirror 19f, a beam detector 19g, etc., and is modulated based on a second image signal VDO2 sent from an external device 42, which will be described later.
is deflected by a polygon mirror 19b rotated at a constant speed by a scanner motor 19a, and horizontally scanned via an imaging lens 19d and folding mirrors 19e and 20 onto a photosensitive drum 21 rotating at a predetermined speed. The reflecting mirror 19f is provided near the end of the scanning area of the polygon mirror 19b in the drum center axis direction, receives the laser beam LB2 deflected and scanned by the polygon mirror 19b immediately before writing an image, and converts the received laser beam LB2 into Lead to beam detector 19g.

The beam detector 19g detects the laser beam LB2 reflected by the reflecting mirror 19f, and detects the laser beam LB2 reflected by the reflecting mirror 19f.
A horizontal synchronizing signal BD2 that determines the image writing timing in the main scanning direction is output to an external device 42 (described later) that serves as an image data supply source, and a second developed color (black ) The second image signal VDO2 for forming an electrostatic latent image to be developed is sent to the printer control unit 4.
1 and is modulated based on the second image signal VDO2, which is emitted from the semiconductor laser 19c. 22 is a first developing device, which emits a laser beam LB.
A black developing material (toner) for developing the electrostatic latent image formed on the photosensitive drum 21 by I into black is stored therein. 23
is a second developing device, which contains a red developing material (toner) for developing the electrostatic latent image formed on the photosensitive drum 21 into red by the laser beam LB2. Reference numeral 24 denotes a transfer sheet leading edge detection sensor, and this transfer sheet leading edge detection sensor 24 detects the transfer sheet P fed by the drive of the registration rollers 17.
When the tip of the registration roller 17 is detected, a stop command to stop the rotation of the registration roller 17 is output to a printer control section (not shown). Reference numeral 25 denotes a transfer charger that transfers the developed image on the photosensitive drum 21 onto the transferred transfer sheet P.

A cleaner section 26 collects toner remaining on the photosensitive drum 21. A primary charger 27 charges the photosensitive drum 21 uniformly. A transport belt 28 transports the transfer sheet P on which developing materials (black and red developing materials) are placed to a position where a fixing roller 29 is provided.

Reference numeral 30 denotes a paper ejecting roller that ejects the fixed transfer sheet P onto a paper ejecting tray 31.

FIG. 5 is an I10 relationship diagram illustrating the interface between the printer body 11 shown in FIG. 4 and external equipment, and 41
is the printer control unit, which is the printer main body 11 shown in FIG.
It is set in. 42 is an external device, the first and second
It functions as a data supply source (host) that sends the image signals VDO1 and VDO2 to the printer control unit 41.

In this figure, VSREQ is a sub-scanning direction synchronization request signal, and a detection signal output when a transfer sheet leading edge detection sensor 24 provided in the printer main body 11 detects the leading edge of the transferred transfer sheet P is sent to the printer control unit 41. is output to the external device 42. VSYNC
is a sub-scanning direction synchronization signal, which is sent back to the printer control section 41 by the external device 42 when the sub-scanning direction synchronization request signal VSREQ is sent and received from the printer control section 41. BD
(including BDI or BD2) is a horizontal synchronization signal, which is sequentially output from the beam detectors 18g and 19g shown in FIG. 4 to the external device 42.

VDO is an image signal (first image signal VDOI and second image signal
(including the image signal VDO2), and the sub-scanning direction synchronization signal V
After sending out the SYNC, the horizontal synchronizing signal BD (first and second horizontal synchronizing signal B) is sequentially output from the printer control unit 41.
DI, BD2) is output to the semiconductor laser 18c or semiconductor laser 19c of the printer body 11, and the first
Laser beams LBI and LB2 modulated based on the image signal VDO1 and the second image signal VDO2 are scanned onto the photosensitive drum 21.

First, the image alignment operation of the photosensitive drum 21 in the main scanning direction will be described.

Laser beam LBI reflected by reflection mirror 18f
is guided by the beam detector 18g, and the horizontal synchronizing signal BDI, which is obtained by waveform-shaping the output of the beam detector 18g at a preset threshold level, is output from the printer control unit 41 to the external device 42. The external device 42 starts sending the image signal, that is, the first image signal VDO1, to the printer control section 41 at a certain timing based on the horizontal synchronization signal BDI output from the printer control section 41. The first image signal VDOI is sent to the image processing means and laser drive circuit (both not shown) of the printer main body 11 via the printer control unit 41, and drives the semiconductor laser 18c.

An electrostatic latent image is formed from a constant position in the direction of the central axis of the photosensitive drum 21 that rotates one line of image information at a predetermined speed. This electrostatic latent image is developed, for example, by jumping, using a developing material (black toner) stored in the first developing device 22, and is visualized.

Similarly, the laser beam LB2 reflected by the reflection mirror 19f is guided to the beam detector 19g, and a horizontal synchronizing signal BD2 obtained by shaping the output of the beam detector 19g at a preset threshold level is sent to the external device 42 from the printer control unit 41. Output to. The external device 42 outputs an image signal,
That is, the printer control unit 41 of the second image signal VDO2
Start sending to. The second image signal VDo2 is sent via the printer control section 41 to the image processing means and laser drive circuit (both not shown) of the printer main body 11, and drives the semiconductor laser 19c. An electrostatic latent image is formed from a constant position in the direction of the central axis of the photosensitive drum 21 that rotates one line of image information at a predetermined speed. A developing material (red toner) in which this electrostatic latent image is accommodated in the second developing device 23
Then, for example, it is developed by jumping and visualized. In this way, the horizontal synchronization signals BDI and BD2 cause synchronization in the main scanning direction, that is, the image positions of the photosensitive drum 21 in the drum center axis direction match.

Next, image alignment in the sub-scanning direction will be explained.

The transfer sheet P stored in the paper feed cassette 12 is sent out from the paper feed cassette 12 by the drive of the pickup roller 13, and is further transferred to the registration roller 17 (in a stopped state) via the conveyance guide 15 by the drive of the rear feed roller 14. It is fed to the set location. At this time, when the leading edge of the transferred transfer sheet P is detected by the pre-registration sensor 16, the driving of the feeding roller 14 is stopped after a predetermined time has elapsed. Therefore, after the pre-registration sensor 16 detects the leading edge of the transferred transfer sheet P, the feeding roller 14
By the time the transfer sheet P stops, the transfer sheet P comes into contact with the stopping registration roller 17, forms a loop within the feeding path, and stops. Thereby, the skew of the transfer sheet P can be corrected.

The transfer sheet P whose skew has been corrected is transferred to the registration roller 17
A transfer sheet leading edge detection sensor 24 is located in the path between the registration roller 17 and the photosensitive drum 21. The driving of the registration rollers 17 is stopped at the moment the leading edge of the transfer sheet P is detected. Next, the photosensitive drum 21
At the timing when the developed image formed on the transfer sheet P is transferred to a predetermined position in the transport direction, the registration roller 1 is moved again.
7 is driven and begins to feed the transfer sheet P toward the photosensitive drum 21. Thereafter, the developed toner image is transferred onto the photosensitive drum 21 on the transfer sheet P by the transfer charger 25, and the transfer sheet P is conveyed to the fixing roller 29 by the conveyance belt 28. The fixing roller 29 fixes the toner image placed on the transfer sheet P by heat and pressure, and the transfer sheet P that has undergone the fixing operation is discharged to the paper discharge tray 31 by the drive of the paper discharge roller 30.

Incidentally, the synchronization of the image position in the sub-scanning direction, that is, the synchronization of the rotational direction of the photosensitive drum 21, is controlled as follows.

When the leading edge of the transfer sheet P is detected by the transfer sheet leading edge detection sensor 24, the driving of the registration rollers 17 is temporarily stopped, and writing of a predetermined image in the transport direction of the transfer sheet P where the image on the photosensitive drum 21 is stopped is started. As described above, re-conveying the image at the timing of transfer to the position will be explained in more detail.

When the transfer sheet leading edge detection sensor 24 detects the leading edge of the fed transfer sheet P by driving the registration rollers 17, this detection signal is sent to the printer control section 41, and the printer control section 41 performs the following operations based on this detection signal. External device 4
A sub-scanning direction synchronization request signal VSREQ is sent to the sub-scanning direction synchronization request signal VSREQ. The external device 42 receives this sub-scanning direction synchronization request signal VSREQ.
When the sub-scanning direction synchronization signal VSYNC is sent and received, the sub-scanning direction synchronization signal VSYNC is sent back to the printer control section 41, and the image signal VDO is sent back to the printer control section 41 in synchronization with the horizontal synchronization signal BD sent from the printer control section 41.

At this time, when the printer control unit 41 receives the sub-scanning direction synchronization signal VSYNC from the external device 42, the printer main body 1
Start various sequences within 1. For example, the printer control unit 41 receives the sub-scanning direction synchronization signal VSYN from the external device 42.
When C is received, a timer included in the printer control section 41 calculates the time and drives the registration roller 17. That is, when the laser beam LBI emitted from the semiconductor laser 18c irradiates the exposure scanning position a on the photosensitive drum 21 at the timing when the sub-scanning direction synchronization signal VSYNC is received, this exposure scanning position a changes due to the rotation of the photosensitive drum 21. The registration roller 17 is driven at a position where the leading edge of the transfer sheet P to be transferred to the transfer charger 25 coincides with the image writing position of the photosensitive drum 21 in the sub-scanning direction.

However, as shown in FIG. 4, in a laser beam printer that obtains a multicolor multiple image, synchronization with the image formed by the second optical scanning system 19 in the sub-scanning direction is required.

FIG. 6 is a schematic diagram illustrating the synchronization operation in the sub-scanning direction in multicolor multiple image formation, and the same parts as in FIG. 4 are given the same reference numerals. Note that the arrow indicates the photosensitive drum 21
indicates the direction of rotation.

The sub-scanning direction synchronization signal VSYNC for the black image developed by the first developing device 22 from the external device 42 is transmitted to the printer control unit 4.
1, the exposure position of the photosensitive drum 21 at the time of reception is the exposure scanning position a shown in FIG. Therefore, the predetermined time Δt(X/v0
; Vo indicates the circumferential speed of the photosensitive drum 21), the first image signal VDO1 is sent from the external device 42 to the printer control unit 41 in synchronization with the horizontal synchronization signal BD1, and this first image signal (2) A laser beam LBI modulated based on D01 is scanned sequentially onto the photosensitive drum 21.

The electrostatic latent image formed by this rask scanning is developed into a black image B1 by the first developing device 22. In order to form an electrostatic latent image on this black image B1 by moving the red image R1 developed by the second developing device 23 to the exposure start position in synchronization with the exposure scanning position a, conventionally, a rask is placed on the photosensitive drum 21. Relative distance La (described later) between the scanned laser beam LBI and laser beam LB2 in the sub-scanning direction and the circumferential speed V of the photosensitive drum 21. In other words, the timing calculated from the sub-scanning direction synchronization request signal VS sent from the printer control unit 41 to form the black image B1 on the photosensitive drum 21
After a predetermined period of time (La/vo) has elapsed from the sending timing of REQ, a sub-scanning direction synchronization request signal VSREQ for forming the red image R1 is sent to the external device 42, and the red image R1 formation is returned from the external device 42. By returning the sub-scanning direction synchronization signal VSYNC to the printer control unit 41, the external device 42 starts from a position a predetermined amount
The laser beam LB2 modulated by the second image signal VDO2 sent out from the image signal VDO2 is scanned. In this way, the image leading edge positions of the black image and the red image on the drum surface of the photosensitive drum 21 match, and control is performed so that the respective image writing positions in the drum rotation direction are also matched.

[Problem that the invention seeks to solve]

However, conventionally, the peripheral speed of the photosensitive drum 21 is V.

The exposure timing of the red image R1 to the photosensitive drum 21 is determined by assuming that
The exposure start position for each color varies each time an image is formed, which is a serious problem. there were.

This invention has been made to solve the above problems, and includes a first laser beam that is exposed to an exposed image carrier to form an electrostatic latent image that is developed into a first developed color. The exposure start position is in the axial direction of the exposure image carrier that is driven to the exposure start position of the second laser beam that exposes the exposure image carrier to form an electrostatic latent image that is developed into a second development color. By detecting the driving distance in the orthogonal sub-scanning direction, the exposure start position of the first laser beam can be precisely matched with the exposure start position of the subsequent laser beam, resulting in clear printing without deviation in the sub-scanning direction. The object of the present invention is to obtain a laser beam printer capable of forming multiple multicolor images.

[Means for solving problems]

In the laser beam printer according to the present invention, a laser beam irradiation position scanned on a rotating image carrier from one optical scanning system is different from a laser beam irradiation position scanned on the rotating image carrier from another optical scanning system. a movement amount detection means for detecting a predetermined rotational drive amount by which the image carrier is rotated until reaching the beam irradiation position; A timing determining means is provided for determining the image writing timing in the sub-scanning direction of the laser beam scanned from each of the plurality of optical scanning systems.

(Function) In this invention, the B motion detection means scans the rotating image carrier from a laser beam irradiation position scanned by one optical scanning system to the rotating image carrier from another optical scanning system. When a predetermined amount of rotational movement of the image carriers is detected before reaching different laser beam irradiation positions, the timing determining means determines the main scanning of the plurality of image carriers based on the detection output of the movement amount detection means. The image writing timing in the sub-scanning direction of the laser beam scanned by each scanning system of the plurality of optical scanning systems in the sub-scanning direction is determined.

〔Example〕

FIG. 1 is a sectional view illustrating the configuration of a laser beam printer showing an embodiment of the present invention, and the same parts as in FIG. 4 are given the same reference numerals.

In this figure, reference numeral 1 denotes a clock disk constituting the movement amount detecting means of the present invention, which rotates at the same speed as the circumferential speed v0 of the photosensitive drum 21, and has N clock slits SL detected by the clock sensor 2 in a lock circle. They are arranged on concentric circles of the plate 1. Note that laser beam LBI and laser beam LB2
Let α° be the critical angle, α”*N/360° is an integer, and the critical angles α and N are designed in advance so that α”*N/360° is an integer. However, if it is not possible to design it to an integer due to the design, the printer An internal timer of the control unit 41 may also be used.
The printer control unit 41 also serves as timing determination means of the present invention, and is configured to control the sub-laser beam scanned from each of the plurality of optical scanning systems in the main scanning direction of the photosensitive drum 21 based on the detection output of the clock sensor 2. Determine the image writing timing in the scanning direction.

FIG. 2 is a perspective view illustrating the arrangement of the clock disk 1 and clock sensor 2 shown in FIG. 1, and the same parts as in FIG. 1 are given the same reference numerals.

FIG. 3 is a timing chart illustrating the image signal transmission timing according to the present invention, in which the same parts as in FIGS. 1 and 2 are given the same reference numerals. However, La is the first
laser beam LBI and second laser beam LB2
indicates the raster scan interval.

When the external device 42 serving as a host receives a ready signal from the print control unit 41, it transmits the print signal to the print control unit 4.
1 and the print sequence is started.

First, the pickup roller 13 is driven to feed out the transfer sheet P stored in the paper feed cassette 12, and the transfer sheet P is sequentially conveyed via the conveyance guide 15 by the drive of the feed roller 14. When the leading edge of the transfer sheet P is detected by the transfer sheet leading edge detection sensor 24, the print control unit 41 recognizes that the transfer sheet P has reached the position where the registration rollers 17 are disposed. The transfer sheet P is further conveyed for a predetermined period of time, and is once stopped in a state in which it contacts the registration rollers 17 and forms a loop, thereby removing the skew. Next, when the registration rollers 17 are driven and the transfer sheet P is detected by the transfer sheet leading edge detection sensor 24, the transfer sheet P is stopped again.

On the other hand, upon receiving and receiving a print start signal sent from the external device 42, the printer control unit 41 outputs a drive command for driving the photosensitive drum 21 to a drum motor (not shown), and rotates the photosensitive drum 21 in the direction of the arrow. . Next, when the first clock signal CLK is generated from the clock sensor 2 after the transfer sheet P comes to a halt,
A vertical synchronization request signal VSREQI triggered using this clock signal CLK as a reference is output to the external device 42.

The external device 42 that has sent and received the vertical synchronization request signal VSREQI immediately returns a sub-scanning direction synchronization signal (vertical synchronization signal) VSYNCI to the printer control unit 41. Based on this sub-scanning direction synchronization signal VSYNC1, the printer control unit 41 executes a print sequence, and the external device 42 outputs the horizontal synchronization signal BDI and vertical synchronization signal VSYN output from the beam detector 18g of the printer main body 11.
First image signal (black image signal) VDOI in synchronization with CI
is transmitted to the printer control unit 41, and this first image signal V
The semiconductor laser 18c is modulated based on DO1, horizontally scans in the main scanning direction on the photosensitive drum 21 rotating in the direction of the arrow, and forms an electrostatic latent image that is developed without an image by the first developing device 22 at the subsequent stage. do.

This operation is repeated sequentially, and after the clock sensor 2 generates the first clock signal CLK and outputs the vertical synchronization request signal VSREQ1 to the external device 42, the clock sensor 2 generates (α°*N/360°) clock signals. When the clock signal CLK is detected, that is, when the black image exposure start position a of the photosensitive drum 21 reaches the irradiation position of the laser beam LB2 irradiated from the semiconductor laser 19c,
The printer control unit 41 transmits a vertical synchronization request signal VSREQ2 to the external device 42, and the external device 4 receives the vertical synchronization request signal VSREQ2.
2 immediately returns a vertical synchronizing signal VSYNC2 to the printer control unit 41, and after receiving this vertical synchronizing signal VSYNC2, a second image is generated in synchronization with a second horizontal synchronizing signal BD2 output from the beam detector 19g of the printer main body 11. A signal (red image signal) VDO2 is sent to the printer control unit 41, and the semiconductor laser 19c is modulated based on this second image signal VDO2, causing the photosensitive drum 2 to rotate in the direction of the arrow.
1 is horizontally scanned in the main scanning direction from a red image exposure start position that coincides with the black image exposure start position a to form an electrostatic latent image that is developed into a light image by the second developing device 23 at the subsequent stage.

In this way, when the black image writing reference position, that is, the vertical synchronization signal VSYNC1 is generated, the sub-scanning position on the photosensitive drum 21 that is irradiated by the semiconductor laser 18c to form a black image forms a red image. In order to do this, it becomes possible to scan the photosensitive drum 21 with the laser beam LB2, which is developed into a red image by detecting the state in which the semiconductor laser 19c reaches the sub-scanning start position. The image writing reference position in the sub-scanning direction can be matched with the image, and a multiplexed multicolor image without color shift can be formed.

In addition, in the above embodiment, if the critical angle between the laser beam LBI and the laser beam LB2 is α°, α.

*We have explained the case where the optical system and clock disk 1 are configured so that N/360° is an integer, but the interval between the slots SL of the clock disk 1 is designed to satisfy the following formula (1). Then, N=360/α
...... (1) However, N is an integer.

The interval between the clock slits SL of the clock disk 1 is a linear angle α
Therefore, this clock disk 1 is connected to the photosensitive drum 21.
When rotating at the same speed as , the control up to the irradiation of the laser beam LBI by the first image signal VDO1 is the same, but the sending timing of the second image signal VDO2 is determined by the clock sensor 2 clock slit SL
It is sufficient to synchronize with the second clock signal CLK outputted from the clock sensor 2 immediately after the first clock signal 'cLK detected and outputted. Needless to say, the positions of the two can be matched, and the control procedure of the control system can be simplified.

〔Effect of the invention〕

As explained above, the present invention provides a method in which a laser beam irradiation position scanned on a rotating image carrier from one optical scanning system is different from a laser beam irradiation position scanned on the rotating image carrier from another optical scanning system. a movement amount detection means for detecting a predetermined amount of rotational movement of the image carrier until it reaches the beam irradiation position; Since the timing determining means for determining the image writing timing in the sub-scanning direction of the laser beam scanned from each scanning system of the plurality of optical scanning systems is provided, the circumferential speed of the drum can be adjusted by changing the load of the motor that drives the photosensitive drum. Even if the first and second
Since the sub-scanning start position of the laser beam can be precisely matched based on the image signal of , it has an excellent advantage that a clear multi-color image without positional deviation in the sub-scanning direction can be formed.

[Brief explanation of drawings]

FIG. 1 is a sectional view illustrating the configuration of a laser beam printer showing an embodiment of the present invention, FIG. 2 is a perspective view illustrating the arrangement of the clock disk and clock sensor shown in FIG. 1, and FIG. 3 is a timing chart explaining the image signal sending timing according to the present invention, FIG. 4 is a sectional view explaining the configuration of a multiple printing type laser beam printer, and FIG. 5 is a diagram showing the printer main body and external equipment shown in FIG. 4. FIG. 6 is a schematic diagram illustrating the synchronization operation in the sub-scanning direction in multicolor multiple image formation. In the figure, 1 is a clock disk, 2 is a clock sensor, 18 is a first optical scanning system, 19 is a second optical scanning system, 22 is a first developer, and 23 is a second developer. Figure 2 Figure 3

Claims (1)

[Claims] A plurality of developing units sequentially develop electrostatic latent images that are sequentially formed on the image carrier based on a plurality of laser beams scanned from a plurality of optical scanning systems to different positions of the image carrier rotating in one direction. In a laser beam printer that forms multiple images on the image carrier by using a laser beam printer, a laser beam irradiation position scanned from one optical scanning system to the rotating image carrier is applied to the rotating image carrier from another optical scanning system. a movement amount detection means for detecting a predetermined rotational movement amount by which the image carrier rotates until reaching a different laser beam irradiation position scanned with respect to the image carrier; and timing determining means for determining image writing timing in the sub-scanning direction of the laser beam scanned by each scanning system of the plurality of optical scanning systems in the main-scanning direction of the image carrier. printer.