A magnetic force estimating unit (38) for estimating the magnetic force of a permanent magnet of a rotor of a motor (6), a determining unit (39), and a demagnetization responsive timing changing unit (40) are provided in an inverter device (22) or an electric control unit (21). The magnetic force estimating unit (38) performs a determination of the magnetic force with a predetermined rule from detection signals of at least two of the motor rotation number, motor voltage and motor current. The determining unit (39) determines whether or not a demagnetization occurs. In response to the result of determination by the determining unit (39), the demagnetization responsive timing changing unit (40) changes the timing, at which the maximum electric current relative to the phase of the rotor is fed, so that with respect the motor drive by the inverter device the reluctance torque of the motor may be increased.