A fixing device includes an endless fixing belt, fixing-belt holding members which hold the fixing belt at opposite ends, a pressure member to be brought into contact with the fixing belt, a nip forming member arranged inside the fixing belt to form a nip area by contacting the pressure member with the fixing belt, a nip supporting member, side plates where the fixing-belt holding members and the nip supporting member are fixed, and a pressing mechanism which presses the pressure member against the nip forming member. The nip forming member includes at least three layers including a heat absorbing layer contacting the nip supporting member. With no load applied, the nip supporting member assumes a shape protruding at longitudinal center toward the pressure member and is in contact with the nip forming member while having the shape protruding at the center portion.