A fusing device for fusing a toner image by passing a recording sheet carrying an unfused toner image by means of the torque of a heating roll through a nip portion between the rotating heating roll and a heat-resistant solid elastic member which is forced to contact the outer peripheral surface of the heating roll to form the nip portion. In the fusing device, the heat-resistant solid elastic member includes an elastic body penetrated with a liquid release agent, and a lease-agent penetration control film whose region which is used for covering the surface of the elastic body and corresponds to at least the nip portion is formed of a porous film.