A fixing device according to the invention comprises a first belt in an endless form, a first heat source for heating the first belt, a first reflection member reflecting heat from the first heat source toward the first belt, and a first temperature detecting member for detecting temperature of the first reflection member by contacting the first reflection member, wherein the first reflection member is disposed between the first heat source and the first temperature detecting member. The fixing device is able to accurately detect the temperature of the member heated by the heat source.