A fixing device includes a cylindrical film including a heat generating layer, a first temperature detecting member contacting the film, and a second temperature detecting member contacting the film and provided at such a position that a temperature change at the position, at which the second temperature detecting member is provided, is slower in responsiveness than at a position at which the first temperature detecting member is provided. A controller stops supply of electrical power to the film depending on a difference between a detection temperature value output by the first temperature detecting member, and a detection temperature value output by the second temperature detecting member. In addition, a toner image, formed on a recording material, is heated by heat from the film and is fixed on the recording material.