The purpose of the present invention is to discharge a capacitor using a motor without rotating the motor even when the capacitor is connected to a position-sensorless motor. A motor drive device calculates a voltage pulse width that does not cause a motor (8) to rotate or vibrate by using a voltage between both ends of a smoothing capacitor (5) and an inductance estimation value for the motor (8), and calculates an inductance by applying a voltage to the motor (8) using the voltage pulse width. Thereafter, the charge stored in the smoothing capacitor (5) is discharged by estimating a rotor position using the inductance calculated for each switching pattern, and allowing a motor current to flow on the basis of the estimated rotor position.