An endless belt, a heat source, a rotatable member forming a nip together with a belt, a nip forming member which includes a first member having a U-shape made of metal and open on a side opposite from the rotatable member and which includes a second member provided inside the first member, a contact member contacting an inner peripheral surface of the belt, a reflecting portion for reflecting, toward the inner peripheral surface of the belt, radiation heat from the heat source toward the nip forming member, and a heat conducting portion, contacting the contact member, for conducting heat of the reflecting portion to the contact member, and a heat insulating member provided between the first member and the second member are provided, and a thickness t (μm) and thermal conductivity λ (W/m·K) satisfy: t≥100 (μm) 0.02 (W/m·K)≤λ≤0.05 (W/m·K).