Provided is an image forming system including a first image forming device, a second image forming device; and an image reading device. A first hardware processor of the first image forming device causes a first image former of the first image forming device to form a first reference image on a first face of a sheet. A second hardware processor of the second image forming device causes a second image former of the second image forming device to form a second reference image different from the first reference image on the first face of the sheet. A third hardware processor of the image reading device determines whether an image positional deviation is present based on the first reference image and the second reference image that are formed on the first face of the sheet and that are read by the image reading device.