An electric motor drive device 1000 includes: an inverter 3 that drives a motor 4, a voltage division circuit 2 that serves as a neutral point potential detection unit that detects a neutral point potential of a stator winding of the motor 4; and a controller 1 that estimates a rotor position of the motor 4 based on the detected neutral point potential, and that controls the inverter 3 based on an estimation result. A ground potential of the controller 1 is set to a negative side potential or a positive side potential of a DC voltage that is supplied to the inverter 3. The voltage division circuit 2 detects the neutral point potential with reference to the negative side potential or the positive side potential. The controller 1 estimates the rotor position based on a difference between a first neutral point potential detected during the ON/OFF operation of the inverter 3 and a first fixed reference potential (⅔)Emax, and based on a difference between a second neutral point potential detected during the ON/OFF operation and a second fixed reference potential (⅓)Emax.