Image data is inputted from a data processing means 123 into storage means 124 so that light emitting elements of one line 128a of a light-emitting element (yellow) line head 128 are activated to expose pixels on an image carrier according to an output signal form a shift resistor 124a. The image carrier is moved in the direction of arrow X in such a manner that the pixels reach a position corresponding to the light emitting elements in a next line 128b. At this point of time, the image data are transmitted to a shift resistor 124b and then outputted to the line 128b so as to expose the pixels again. The image data are transmitted among the shift resistors sequentially by moving the image carrier, thereby sequentially repeatedly expose the same pixels.