The present invention aims to effectively perform image data transfer between an image generation controller and an image processing controller in either of color image printing and black-and-white image printing. Respective signal lines of Y, M, C and K are provided in a video interface formed between a PDL controller and the image processing controller. When the color image is printed by a printer of one-drum structure, data of a one color for four lines are simultaneously transferred by using the four signal lines, and this transfer operation is repeated for one page and for the four colors. When the black-and-white image is printed, black data for the four lines are simultaneously transferred by using the four signal lines, and this transfer operation is repeated for one page.