A fixing device for thermally fixing a developer image to a sheet fed in a sheet feeding direction includes: a tubular flexible fusing member; a heater; a nip member; a resiliently deformable backup member; and an adjustment mechanism. The 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 resiliently deformable backup member provides a nip region in cooperation with the nip member upon nipping the fusing member between the backup member and the nip member. The adjustment mechanism moves the nip member between a first position and a second position different from the first position in the sheet feeding direction to adjust the sheet feeding direction.