A fixing device includes an endless, flexible fixing member, a metal member, a heater, a pressing member, a temperature detector, and a supporting member. The fixing member is rotatably provided in the fixing device to heat a toner image thereon. The metal member is fixedly mounted in the fixing device so as to be opposite an inner circumferential surface of the fixing member, to maintain the fixing member in a substantially circularly loop shape. The heater is disposed near the metal member to heat the metal member. The pressing member is rotatably pressed against an outer circumferential surface of the fixing member to form a nip portion between the pressing member and the fixing member. The temperature detector is disposed in contact with the metal member, to detect a temperature of the metal member. The supporting member is disposed between the heater and the temperature detector to support the temperature detector.