A fixing device includes a fixing unit, a temperature sensor, and a controller. The fixing unit includes a fixing member with which a medium on which a developer image is transferred comes into contact and a heat source that supplies heat to the fixing member. The temperature sensor measures temperature of a surface with which the medium comes into contact in the fixing member. The controller outputs information indicating an abnormality of temperature detection by the temperature sensor if an estimation value of temperature of the fixing member is larger than a predetermined reference value.