To provide a fixing unit with built-in thermostat which comprises a small number of components, can inexpensively be manufactured, and can perform temperature detection with high thermal response property.

The fixing unit with built-in thermostat is equipped with a ceramic heater 1, a holder 3 which holds the heater 1, a thermostat part 5 incorporated in the holder 3, and a fixing film 63 into which the holder 3 is rotatably inserted. The thermostat part 5 has a first terminal 7 and a second terminal 9 fixed to the holder 3, a fixed contact 11 provided to the first terminal 7, a movable contact 13 which contacts or moves away from the fixed contact 11, a movable spring 15 which energizes the movable contact 13 to the fixed contact 11, and a bimetal 19 which performs inverting operation at set upper-limit temperature to move a guide pin 17 upward and separate the movable contact 13 from the fixed contact 11 against the energizing force of the movable spring 15. The bimetal 19 is arranged in a through hole 25 bored in a bottom wall of the holder 3 and abuts against a top surface of the heater 1.