A ROM (18) stores image forming driver information associated with a plurality of size groups into which a plurality of sets of sheet sizes close to each other are respectively sorted. When a sheet size of an image transfer sheet designated by an operator through an operating section (19) and a sheet size of an image transfer sheet detected by an image transfer sheet sensor (21) are different from one another, but both of the image transfer sheet sizes fall in the same size group, a CPU (15) allows an image forming section (16) to execute an image forming processing in accordance with image forming driver information corresponding to the image transfer sheet size designated by the operating section (19).