There is provided a fixing device for thermally fixing a developing agent image to a sheet. The fixing device includes: 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 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 providing 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 to detect a temperature of the reflection plate.