Disclosed is a laser printer for forming an image by transferring images one after another to a printing medium from a plurality of photosensitive drums disposed along the transport path of the printing medium. The printer includes data processing units provided in correspondence with the drums in a number smaller than the number of drums, each of the data processing units processing an image signal and generating image data for image formation, and data transfer units, which are provided in correspondence with respective ones of the data processing units, for supplying the image forming units with image data that has been generated by the data processing units. If the length of the printing medium along the medium transport path is less than the overall length along which the plurality of drums are disposed, decoders and FIFO buffers are assigned commonly to one video interface and video data generated by these decoders is output to corresponding ones of video interfaces via the FIFO buffers.