A fixing device includes an endless fixing belt which is tensionless in a circumferential direction in a center part in a lateral direction and supported to circularly move, a heating device that heats the fixing belt, a pressure roll contacted with an outer peripheral face and supported to rotate around an axis, a pressure member contacted with an inner peripheral face to exert pressure on the pressure roll, and a support member passed through inside the fixing belt and supporting the pressure member. Preferably, an unfixed toner image on a recording medium passing the nip part which is formed between the fixing belt and the pressure roll is pressurized and heated to be formed into a fixed image, and the fixing belt is kept in a round shape in the circumferential direction near both side edges and supported to rotate around a center axis determined by the support member.