An image-data transfer controller divides one line of image data for each of the light-emitting-element array units, transfers the divided image data to respective light-emitting-element array units, to drive each of the light-emitting elements in the light-emitting-element array units for an exposure in a main-scanning direction. The image-data transfer controller divides the one line of image data for each of the light-emitting-element array units and transfers the divided image data during one-line interval in the main-scanning direction, performs a plurality of data transfers in the one-line interval for driving each of the light-emitting elements by controlling a lighting time, and includes a plurality of lines of storing units.