A fixing device includes a fixing roller, a heat roller, an endless fixing belt wound around the two rollers to be driven to rotate, a nip forming member to form a nip while contacting the fixing belt at a position opposite the fixing roller, a pressing member to press the heat roller to separate it from the fixing roller, and a temperature detector to detect a temperature of the fixing belt at a position opposite the heat roller. The temperature detector is provided opposite or in contact with the fixing belt at a position, in parallel with the moving direction of the heat roller, on the outermost periphery of the displacement area of a surface of the fixing belt which displaces according to the approaching and departing movement of the heat roller. With such a simple structure, the temperature detector can detect the temperature of the fixing belt accurately.