A measuring section that measures the position shift in the main scanning direction of a plurality of light sources, a signal generating section that generates synchronized INDEX signals that in which the writing timing of the different light sources has been adjusted according to the position shifts, a signal generating section that generates the signals for driving the light sources based on the synchronized INDEX signals and image data, and a measuring section that detects the timing shift component in the main scanning direction included in the synchronized INDEX signal generated in the condition in which adjustments have been made according to the position shift, and further shift correction is carried out in the main scanning direction based on the timing shift component, for the synchronized INDEX signal which are adjusted and generated in accordance with the position shift of a plurality of light sources.