PURPOSE: To so control the voltage of a main circuit smoothing capacitor as to hold constantly by supplying almost no leakage current of regenerative energy to a regenerative reference voltage smoothing capacitor, shifting the potential difference of the capacitor to be generated by the leakage current, and inputting it to a comparator.

CONSTITUTION: A regenerative circuit 18 having a regenerative operation transistor 14 does not supply a leakage current of regenerative energy to a regenerative reference smoothing capacitor 13 when the positive or negative electrode side potentials of a main circuit smoothing capacitor 3 and a regenerative reference voltage smoothing capacitor 7 are equal, and the potential difference of the capacitors 3, 7 generated by the leakage current is directly input to a comparator 10. A switching element 9 is turned ON, and the regenerative energy is consumed by a regenerative energy consuming resistor 8 connected in parallel with the capacitor 3. Thus, the energy can be so controlled as to hold the voltage of the capacitor 3 constant.