A fixing device for thermally fixing a developing agent image to a sheet fed in a sheet feeding direction includes: a tubular flexible member; a nip member; a heater; a reflection member; a backup member; and a temperature sensor. The tubular flexible member has an inner peripheral surface defining an internal space. The nip member is disposed in the internal space and has one surface and opposite surface. The inner peripheral surface is in contact with the one surface. The heater is disposed in the internal space and confronts the nip member in a confronting direction. The reflection member is configured to reflect a radiant heat from the heater toward the nip member. The backup member is configured to provide a nip region in cooperation with the nip member for nipping the tubular flexible member between the backup member and the nip member. The temperature sensor is disposed in the internal space and is configured to detect a temperature of the nip member. The temperature sensor is positioned outside of the reflection member and in confrontation with the opposite surface.