To provide a thermal fixing device having a surface temperature detection means constituted so that the increase of friction with a movably heating member may prevented by uniformizing the pressure distribution at a contact part to the surface shape of the movable heating member such as a roller or a belt whose surface is soft.

This thermal fixing device is provided with a thermistor unit provided with a thermistor 501, a heat resistant sponge 502 elastically holding the thermistor 501, a polyimide film 506 insulating the thermistor 501 from a roller 401 and also conducting heat to the thermistor 501 while sliding on the surface of the roller 401, and a pressure spring 504 integrally holding and pressing the thermistor 501, the sponge 502 and the film 506 to the roller 401. Relating to the device, the thermistor unit is provided with hardening resin 101 which is solidified and formed along the surface shape of the roller 401 so as to integrally fix the thermistor 501 on the sponge 502.