A fixing device shifts to the anomaly detection mode in arbitrary timing except during printing. In the anomaly detection mode, a first heat generation member and a second heat generation member are off and only a third heat generation member is turned on for a predetermined time. A quantity of temperature change in a fixing member during that time is detected by a first temperature detection member or a second temperature detection member. When the quantity of temperature change is within a predetermined range, it is determined that the third heat generation member is normal. In inspecting whether the third heat generation member is normal or not, when the temperature of the fixing member is in the area exceeding T1° C. where the temperature rise gradient of the fixing member is gentler, the inspection is skipped.