An image-data transfer controller divides one line of image data for each of a plurality of light-emitting-element array units, transfers the divided image data to respective light-emitting-element array units, to drive light-emitting elements in the light-emitting-element array units for an exposure in a main-scanning direction. The image-data transfer controller performs a plurality of data transfers during one-line interval in the main-scanning direction, while performing a data processing of the one line of image data in the respective light-emitting-element array units, to drive a light-emitting element array formed with the light-emitting elements arranged in one direction.