To reduce the size and weight of a regenerative-braking resistor and facilitate its use as an on-board power supply, while protecting a capacitor from overvoltage.

A positive-side terminal of a battery 2 and an anode-side terminal of a diode D1 are connected with each other, while one end of a capacitor 3 and a DC input positive-side terminal of an inverter 5 are connected to a cathode-side terminal of the diode D1, to allow the diode D1 to prevent charging to the battery 2 by regenerative power from a motor 4. The motor 4 is connected to the output side of the inverter 5, and the other end of the capacitor 3 is connected with a negative-side terminal of the battery 2. The DC input negative-side terminal of the inverter 5 is also connected with the negative-side terminal of the battery 2; a series circuit of a MOSFET 9 and a regenerative-braking resistor r1 is connected to the diode D1 in parallel; and the anode-side terminal of a Zener diode 10 is connected to the gate terminal of the MOSFET 9, while a cathode-side terminal of the Zener diode 10 is connected to the cathode-side terminal of the diode D1.