A main transistor and a reverse current prevention transistor are provided in series between an input terminal and an output terminal. An input diode is provided between a connection point of the reverse current prevention transistor and the main transistor and a reference voltage terminal in such a direction that the anode becomes the reference voltage terminal side. A control unit controls the gate voltage of the main transistor according to a DC voltage. The reverse current prevention transistor is arranged in such a direction that the anode of its body diode becomes the input terminal side. The reverse current prevention transistor is biased to be turned on in a normal state that the input terminal becomes high potential and the reference voltage terminal becomes low potential.