A fixing device includes: a rotatable endless belt type fixing member; a pressurizing member that is opposed to the fixing member and rotates; and a nip forming member that is arranged inside the fixing member, and forms a nip part by contact with the pressurizing member through the fixing member. The nip forming member includes: a heat-uniformizing member having bent portions opposed to each other; a heat-insulating member arranged inside the heat-uniformizing member; a heat-absorbing member arranged on an upper surface of the heat-insulating member; and a sliding sheet that covers a nip side of the heat-uniformizing member, and is held between the bent portions of the heat-uniformizing member and the heat-insulating member. The heat-uniformizing member and the heat-insulating member are fastened to each other by fitting a fitting part formed on the heat-uniformizing member to a fitted part formed on the heat-insulating member.