A fixing device for thermally fixing a developing agent image to a sheet fed in a sheet feeding direction including: 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, disposed in the internal space, is configured to radiate radiant heat. The nip member, disposed in the internal space, is configured to receive the radiant heat. The inner peripheral surface makes 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 adjustment mechanism is configured to move the backup 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.