When the current flowing through each electric terminal of an AC motor 21 reaches the vicinity of zero, an operation putting the electric terminals of the AC motor 21 into an opened state, or putting an electric terminal into a conductive state via a reflux diode inside an inverter 11, is carried out. Herein, as an operation such that the current flowing through each electric terminal reaches the vicinity of zero, the electric terminals are short-circuited by all upper arm or lower arm switching elements of the inverter 11 being turned on. By so doing, a flow of electromagnetic energy of a reactance component of the AC motor 21, from the AC motor 21 into the inverter 11 side when the drive of the inverter 11 is stopped, is prevented or suppressed. As a result of this, an overvoltage or overcurrent is prevented.