In a motor temperature estimating device including: a polyphase motor 1 having a first phase coil, a second phase coil and a third phase coil; current sensors; a determination section that determines whether or not the polyphase motor is rotating; and a transformation section that transforms a first phase output signal from the first phase current sensor, a second phase output signal from the second phase current sensor, and a third phase output signal from the third phase current sensor to output a post-transformation output signal, the post-transformation output signal is inputted to a first temperature estimation section that estimates the temperature of the polyphase motor during rotation, and the first phase output signal, the second phase output signal and the third phase output signal are inputted to a second temperature estimation section that estimates the temperature of the polyphase motor during stop.