To protect switching devices from breakdown when excessive voltages are applied to the devices, by a method wherein the switching devices are connected so as to have the leakage current of the 2nd switching device larger than the leakage current of the 1st switching device and have the leakage current of the 3rd switching device larger than the leakage current of the 4th switching device.

1st and 2nd switching devices 21 and 22 are connected in series in a forward direction between the (+) side terminal and the output terminal of a DC voltage source 1, and 3rd and 4th switching devices 23 and 24 are connected in series in a forward direction between the (-) side terminal and the output terminal of the DC voltage source 1. A device whose leakage current is larger than the leakage current of the 1st switching device 21 is selected as the 2nd switching device 22, and a device whose leakage current is larger than the leakage current of the 4th switching device 24 is selected as the 3rd switching device 23. When an output voltage is -E, a voltage higher than a potential difference between a neutral point and the (+) side is not applied to the 1st switching device 21. In the same way, when the output voltage is +E, a voltage higher than a potential difference between the neutral point and the (-) side is not applied to the 4th switching device 24.