An image forming device and method are provided which reduce erroneous detection of a border between transfer material areas when forming a new image by using unused portions of partially used transfer material areas. The image forming device forms an image having a first size or a second size which is one-half of the first size or smaller on a recording medium by transferring transfer materials respectively for transfer material areas, while feeding a strip-shaped transfer medium on which the transfer material areas each having the first size and respectively corresponding to the transfer materials are repeatedly arranged in a predetermined sequence in the longitudinal direction thereof. The transfer medium is fed in a forward direction until an unused portion of a subsequent transfer material area adjacent to a used portion of a transfer material area reaches the detection position and thereafter fed in the reverse direction, so that the border between the used and unused portions is detected from the unused portion side, when a second-size image is newly formed by using unused portions in the respective transfer material areas having been used to form a second-size image.