holding member 3 can be improved consequently the accuracy of temperature detection can be improved in comparison with that in the conventional arrangement also because the positioning accuracy is improved the area of the non contact surface of the heater 8 in the through hole 9 c can be reduced relative to that in the conventional arrangement so that the possibility of occurrence of damage to the heater caused by nonuniformity of fixation heating or thermal stress in this embodiment the positioning holes and the positioning projections are provided as positioning portions on the temperature detecting element holding member 3 side and on the heater holding member 9 side respectively needless to say the arrangement may alternatively be such that positioning projections are provided on the temperature detecting element holding member 3 side while positioning holes are provided on the heater holding member 9 side this embodiment has been described with respect to a case where one temperature detecting device is provided for one heat fixing device however the present invention can also be applied to a heat fixing device of such a type that two or more temperature detecting devices are provided for one heat fixing device and temperature adjustment control is performed by detecting the temperatures of different portions with the temperature detecting devices also in such a case the present invention is effective in stabilizing the accuracy of temperature detection at each portion in a case where two or more temperature detecting devices are provided at least one of the temperature detecting devices is arranged in accordance with the present invention if a particularly high accuracy is required or a size restriction is imposed with respect to the place where the temperature detecting device is provided while the temperature detecting.