A fixing device for thermally fixing a developing agent image to a sheet includes: a flexible fusing member; a heater; a nip member; a backup member; and a temperature detection element. The flexible fusing member has an inner peripheral surface defining an internal space. The heater is disposed in the internal space and radiates radiant heat. The nip member is disposed in the internal space and receives the radiant heat from the heater. The inner peripheral surface is in sliding contact with the nip member. The backup member provides a nip region in cooperation with the fusing member upon nipping the fusing member between the backup and nip members. The backup member confronts the nip member in a confronting direction. The temperature detection element is disposed in a superposed region of the nip member superposed with the nip region in the confronting direction and detects a temperature of the region.