A fixing device for thermally fixing a developing agent image to a sheet includes: a tubular flexible fusing member; a heater; a nip member; and a backup member. The tubular 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 member and the nip member. The nip member has a surface confronting the heater. The surface includes a first heat absorption region having a first heat absorption ratio of a predetermined value and a second heat absorption region having a second heat absorption ratio higher than the first heat absorption ratio. The second heat absorption region is positioned at a region of the surface overlapping with the nip region.