There is provided an image forming system that is capable of making uniform in size printed images, which should originally be the same in size, between transfer sheets on which the images have been printed by a plurality of printers having different functions and capabilities when enough time has elapsed after fixing. In each of the printers, an image controller or the like forms predetermined marks on a transfer material, and a fixing device thermally fixes of the predetermined marks formed on the transfer material, and a mark detecting device or the like detects the predetermined marks. A host server connected to the printers via a network adjusts the sizes of images to be formed by the respective printers according to the predetermined marks detected by the respective printers, and then transfers the image data to the printers. Each of the printers performs image formation based on the transferred image data.