An optical writing device comprising: two or more light sources (11, 12) that can be driven independently and are placed away from each other in a sub-scanning direction of a photoreceptor, wherein each distance of beams emitted from the light sources (11, 12) to the photoreceptor is different; a phase control unit (224, 228) that causes a difference in phase among timing control data of two or more beams emitted from the two or more light sources (11, 12) by a phase change amount, wherein the phase change amount is determined on the basis of beam position of the each beam on the photoreceptor in a main scanning direction and varies depending on the beam position; and a modulating unit that modulates the timing control data of the respective beams phase-controlled by the phase control unit (224, 228) by using a same clock; characterized in that: the phase control unit (224, 228) comprises a selector and a shift-amount designating unit; the selector is configured to receive an image signal for continuous two pixels in the main scanning direction; and the optical writing device further comprises: a shift-amount determining unit configured to measure write widths of a first light source and a second light source of the two or more light sources (11, 12) by calculating an interval time which is an interval between an output time of a detection signal output from a first synchronization detection element and an output time of a detection signal output from a second synchronization detection element for each of the first and second light sources (11, 12), and to determine a change width of the shift amount changed in scanning in the main scanning direction on the basis of the difference in the time interval, wherein the shift-amount designating unit changes the phase change amount in accordance with the width of the shift amount determined by the shift-amount determining unit. two or more light sources (11, 12) that can be driven independently and are placed away from each other in a sub-scanning direction of a photoreceptor, wherein each distance of beams emitted from the light sources (11, 12) to the photoreceptor is different; a phase control unit (224, 228) that causes a difference in phase among timing control data of two or more beams emitted from the two or more light sources (11, 12) by a phase change amount, wherein the phase change amount is determined on the basis of beam position of the each beam on the photoreceptor in a main scanning direction and varies depending on the beam position; and a modulating unit that modulates the timing control data of the respective beams phase-controlled by the phase control unit (224, 228) by using a same clock; the phase control unit (224, 228) comprises a selector and a shift-amount designating unit; the selector is configured to receive an image signal for continuous two pixels in the main scanning direction; and the optical writing device further comprises: a shift-amount determining unit configured to measure write widths of a first light source and a second light source of the two or more light sources (11, 12) by calculating an interval time which is an interval between an output time of a detection signal output from a first synchronization detection element and an output time of a detection signal output from a second synchronization detection element for each of the first and second light sources (11, 12), and to determine a change width of the shift amount changed in scanning in the main scanning direction on the basis of the difference in the time interval, wherein the shift-amount designating unit changes the phase change amount in accordance with the width of the shift amount determined by the shift-amount determining unit. a shift-amount determining unit configured to measure write widths of a first light source and a second light source of the two or more light sources (11, 12) by calculating an interval time which is an interval between an output time of a detection signal output from a first synchronization detection element and an output time of a detection signal output from a second synchronization detection element for each of the first and second light sources (11, 12), and to determine a change width of the shift amount changed in scanning in the main scanning direction on the basis of the difference in the time interval, wherein the shift-amount designating unit changes the phase change amount in accordance with the width of the shift amount determined by the shift-amount determining unit.