Fixing device provided with temperature sensor

A fixing device for thermally fixing a developing agent image to a sheet comprising:

• a tubular flexible member having an inner peripheral surface defining an internal space;
• a nip member disposed in the internal space and having one surface, the inner peripheral surface being configured to be in sliding contact with the one surface;
• a heater disposed in the internal space:
• a reflection plate configured to reflect a radiant heat from the heater toward the nip member;
• a backup member configured to provide a nip region in cooperation with the nip member for nipping the flexible member between the backup member and the nip member;
• and
• a temperature sensor disposed in the internal space of the tubular flexible member to detect a temperature of the reflection plate, the temperature sensor having a temperature detection surface that faces the reflection plate.