A fixing apparatus of an embodiment of the invention includes an auxiliary pressure member to press a belt to a heat roller side. A belt mechanism to integrally support the belt and the auxiliary pressure member is rotated so that the belt is spaced from the heat roller except for a necessary time. The temperature gradient characteristic of the surface temperature of the heat roller is compared with a reference temperature gradient characteristic, and a correction value of power supplied to a coil is estimated. The correction value is added to the supply power obtained from the detection result of a sensor, and control power for controlling an inverter drive circuit is obtained.