A magnet temperature estimating device for a motor provided with a rotor having magnets and configured to output a rotational motive force, and a stator having a plurality of coils opposing the rotor with an aperture therebetween, is provided. The device includes a sensor configured to detect an induced voltage induced by rotation of the rotor, and a controller configured to control the motor by supplying power to the plurality of coils in response to an input of a detection signal from the sensor. The controller estimates a temperature of one of the magnets based on an amplitude of a frequency spectrum corresponding to a given frequency, among frequency components constituting the induced voltage.