In case of outputting data mixedly including plural different-type images, productivity and cost performance are improved without an operator's complicated assistive operation. To do so, a color printer, a black and white printer and a finisher are electrically connected with others through a data communication path such that these devices can perform data communication with others, and also these devices are mechanically connected with others such that the sheets discharged from the respective printers can be carried to a sheet carrying path of the finisher without any operator's manual working. Further, it is controlled that, in the series of images, pages corresponding to the color images are subjected to image formation by the color printer and pages corresponding to the black and white images are subjected to image formation by the black and white printer, and in the finisher it is selectively set the image formation operation in the color printer and the image formation operation in the black and white printer to be on standby so as to compile the sheets from these printers as one sheet sheaf with the continuous page order maintained.