A power supply device comprises a bidirectional voltage converter unit that performs voltage stepping up or voltage stepping down between a first terminal and a second terminal; a first switching unit that applies a DC voltage obtained from the first electric power supply source to the first terminal; a second switching unit that applies a DC voltage obtained from the second electric power supply source to the first terminal; a third switching unit that applies a DC voltage obtained from the first electric power supply source to the second terminal; an output unit that outputs a predetermined DC voltage from the second terminal; and a switching control unit that controls switching of the first switching unit, the second switching unit and the third switching unit.