A power supply unit has one end on the low potential side, said end being connected to a switching element on the power supply line side, and serves as an operation power supply for outputting a switch signal to the switching element. A switching element includes a first and a second electrode, and renders an electric current in only a direction from the second electrode to the first electrode conducting. A diode is connected in parallel to the switching element while the cathode thereof is directed to a power supply line. A capacitor has one end connected to the first electrode of the switching element and the other end connected to the other end of the power supply unit, and serves as an operation power supply for outputting a switch signal to the switching element.