An image forming device comprising: a feeding unit (4A, 4B, 34) configured to feed a strip-shaped transfer medium on which a plurality of transfer material areas (Y0, Y1, M0, M1,...; C0, C1,...; OP0, OP1,...) each having a first size and respectively corresponding to a plurality of transfer materials are repeatedly arranged in a predetermined sequence in the longitudinal direction of the transfer medium; an image forming unit (3, 9, 33) to form an image having the first size or a second size which is not more than one-half of the first size on a recording medium by transferring the transfer materials respectively for the transfer material areas; a detection unit to detect a border between the transfer material areas based on a change of colors of the transfer material areas which are fed by the feeding unit and pass through a detection position (Ps); and a control unit to control the feeding unit; characterized in that the control unit is configured to control the feeding unit, when a second image of the second size is newly formed by the image forming unit by using unused portions respectively included in the transfer material areas which have been partly used to form a first image of the second size, to feed the transfer medium for each of the partly used transfer material areas in a forward direction until a used portion of the partly used transfer material area passes through the detection position and an unused portion of a next transfer material area reaches the detection position and thereafter to feed the transfer medium in the reverse direction, allowing the detection unit to detect a border between the used portion and the unused portion from the unused portion side. a feeding unit (4A, 4B, 34) configured to feed a strip-shaped transfer medium on which a plurality of transfer material areas (Y0, Y1, M0, M1,...; C0, C1,...; OP0, OP1,...) each having a first size and respectively corresponding to a plurality of transfer materials are repeatedly arranged in a predetermined sequence in the longitudinal direction of the transfer medium; an image forming unit (3, 9, 33) to form an image having the first size or a second size which is not more than one-half of the first size on a recording medium by transferring the transfer materials respectively for the transfer material areas; a detection unit to detect a border between the transfer material areas based on a change of colors of the transfer material areas which are fed by the feeding unit and pass through a detection position (Ps); and a control unit to control the feeding unit; the control unit is configured to control the feeding unit, when a second image of the second size is newly formed by the image forming unit by using unused portions respectively included in the transfer material areas which have been partly used to form a first image of the second size, to feed the transfer medium for each of the partly used transfer material areas in a forward direction until a used portion of the partly used transfer material area passes through the detection position and an unused portion of a next transfer material area reaches the detection position and thereafter to feed the transfer medium in the reverse direction, allowing the detection unit to detect a border between the used portion and the unused portion from the unused portion side.