A fixing device for thermally fixing a developing agent image to a sheet includes a tubular member, a nip plate, a heater, a reflection plate, and a backup member. The tubular member has an inner peripheral surface defining an internal space and an axis defining an axial direction. The nip plate is disposed in the internal space, and the inner peripheral surface is in sliding contact with the nip plate. A heater is disposed in the internal space and confronts the nip plate in a confronting direction. The reflection plate is configured to reflect radiant heat from the heater toward the nip plate. A backup member is configured to provide a nip region in cooperation with the nip plate for nipping the fusing member between the backup member and the nip plate.