An image forming device forms a toner image on a recording medium. The recording medium goes through a fixing nip N between a fixing member and a pressure member in a fixing unit where the toner image is fixed on the recording medium. A fixing temperature detector detects the temperature of the fixing member. A fixing temperature control unit controls a heater that heats the fixing member so that the detected temperature becomes a target temperature. A size control unit of white space sets a white space at a tip of the recording medium in a direction of the recording medium movement. When a paper counter counts more than a predetermined paper count, the size control unit increases the size of the white space. The fixing temperature control unit decreases the target fixing temperature when the white space goes through the fixing nip.