A fixing device for thermally fixing a developing agent image to a sheet fed in a sheet feeding direction comprising: a tubular flexible fixing member having an inner peripheral surface defining an internal space; a nip plate (130, 230) disposed in the internal space and having a first surface and a second surface opposite to the first surface, the inner peripheral surface being in sliding contact with the first surface; a heater configured to generate a radiant heat and disposed in the internal space and confronting the nip plate (130, 230) in a confronting direction: a backup member providing a nip region in cooperation with the nip plate (130, 230) for nipping the fixing member between the backup member and the nip plate (130, 230); and a temperature sensor (170, 570) disposed in the internal space and detecting a temperature of the nip plate (130, 230), characterized by a reflection plate (140, 540) that reflects the radiant heat from the heater toward the nip plate (130, 230), wherein the reflection plate (140, 540) is configured into U-shape in cross-section and surrounds the heater to reflect the radiant heat from the heater towards the second surface of the nip plate (130, 230); and in that the temperature sensor (170, 570) is positioned outside of the reflection plate (140, 540) in the sheet feeding direction and the temperature sensor (170; 570) is disposed on the second surface of the nip plate (130, 230). a tubular flexible fixing member having an inner peripheral surface defining an internal space; a nip plate (130, 230) disposed in the internal space and having a first surface and a second surface opposite to the first surface, the inner peripheral surface being in sliding contact with the first surface; a heater configured to generate a radiant heat and disposed in the internal space and confronting the nip plate (130, 230) in a confronting direction: a backup member providing a nip region in cooperation with the nip plate (130, 230) for nipping the fixing member between the backup member and the nip plate (130, 230); and a temperature sensor (170, 570) disposed in the internal space and detecting a temperature of the nip plate (130, 230), characterized by a reflection plate (140, 540) that reflects the radiant heat from the heater toward the nip plate (130, 230), wherein the reflection plate (140, 540) is configured into U-shape in cross-section and surrounds the heater to reflect the radiant heat from the heater towards the second surface of the nip plate (130, 230); and in that the temperature sensor (170, 570) is positioned outside of the reflection plate (140, 540) in the sheet feeding direction and the temperature sensor (170; 570) is disposed on the second surface of the nip plate (130, 230). a backup member providing a nip region in cooperation with the nip plate (130, 230) for nipping the fixing member between the backup member and the nip plate (130, 230); and a temperature sensor (170, 570) disposed in the internal space and detecting a temperature of the nip plate (130, 230), characterized by a reflection plate (140, 540) that reflects the radiant heat from the heater toward the nip plate (130, 230), wherein the reflection plate (140, 540) is configured into U-shape in cross-section and surrounds the heater to reflect the radiant heat from the heater towards the second surface of the nip plate (130, 230); and in that the temperature sensor (170, 570) is positioned outside of the reflection plate (140, 540) in the sheet feeding direction and the temperature sensor (170; 570) is disposed on the second surface of the nip plate (130, 230).