The present invention minimizes heat-induced damage to a transfer medium when a transfer material region is partially used. An image forming device (1), having: transportation units (4A, 4B) for transporting a strip-shaped transfer medium (4) in which a plurality of transfer material regions (Y, M, C, OP) having a first size and respectively corresponding to a plurality of transfer materials are repeatedly arranged in a predetermined sequence along the longitudinal direction; and an image forming unit (3) for heat-transferring the transfer material for each of the transfer material regions and forming on a recording medium an image that has a first size or a second size that is up to half the size of the first size. When an image having the second size is formed, there is used in the image forming unit either an unused portion included in each transfer material region already used to form an image having the second size or a new transfer material region on the transfer medium. An image having the second size in which a new transfer material region is used is formed at a slower speed than that for forming of an image having the first size.