In a bidirectional power supply device, a DC-DC converter is connected such that the longer an ON time of the first switch becomes, the higher a voltage of a second positive terminal and a second negative terminal becomes. When stopping the supply of power from a first positive terminal and a first negative terminal to a second positive terminal and a second negative terminal, a control circuit turns OFF a third switch, and then operates a switching signal generation circuit so that the ON time of the first switch becomes a maximum. The switching signal generation circuit is operated so that the ON time of the first switch becomes a maximum with the third switch turned OFF in start-up when supplying power from the second positive terminal and the second negative terminal to the first positive terminal and the first negative terminal.