A fixing device including: a heating roller; a pressurizing roller, pressed against the heating roller; a belt extension member, arranged on an upstream side of a medium delivery direction with respect to the pressurizing roller; a belt, wound around outer peripheries of the pressurizing roller and the belt extension member, and moving while forming a fixing nip portion between the heating roller and the belt; and a member, moving the belt extension member rotationally to vary a winding amount of the belt around the heating roller.