The image forming system is provided with: a printer that superposes and transfers plural images formed by use of plural image forming materials to a medium; and a data processor that supplies data of the images to the printer. The data processor includes: a data generation unit that generates data of image components corresponding to the respective kinds of the image forming materials; a data forwarding unit that forwards the generated data to the printer; and an assigning unit that assigns the image components in accordance with the data amount of the forwarded data. The printer includes: an output controller that controls an image forming mechanism to form an image by use of the image forming materials; and a switch processing unit that transmits the data received from the data processor, to the output controller corresponding to the image components of the received data.