A belt fixing unit has a fixing roller and a pressing roller that are provided in a pair midway along a transport passage through which paper having a toner image formed thereon is transported from bottom to top approximately in a vertical direction and that fix the toner image on the paper, a heating roller that is provided parallel to and in a position approximately horizontal to the fixing roller and that incorporates a heater for heating the fixing roller, and a fixing belt that is formed as an endless belt and that is wound around and between the fixing roller and the heating roller. A non-contact temperature sensor for controlling the fixing temperature is provided on a non-contact basis in the paper passage region on the surface of the upper one of the parts of the fixing belt facing each other up and down between the fixing roller and the heating roller.