A power supply device that is adapted to receive supply of electric power from a first electric power supply source (140; 150) or from a second electric power supply source and is adapted to generate a predetermined DC voltage, comprising: a bidirectional voltage converting means, which includes a first terminal and a second terminal, for performing step-up converting or step-down converting between the first terminal and the second terminal; a first switching means for connecting the first electric power supply source (140; 150) to the first terminal of the bidirectional voltage converting means; a second switching means for connecting the second electric power supply source to the first terminal of the bidirectional voltage converting means; a third switching means for connecting the first electric power supply source (140; 150) to the second terminal of the bidirectional voltage converting means; an output means for outputting a DC voltage at the second terminal of the bidirectional voltage converting means as the predetermined DC voltage; and a switching control means for controlling switching of the first switching means, the second switching means and the third switching means, wherein the switching control means is capably of: opening the first switching means, while closing the second switching means and closing the third switching means, for applying a first DC voltage obtained from the first electric power supply source (140; 150) to the second terminal of the bidirectional voltage converting means to transmit electric power to the second electric power supply source via the bidirectional voltage convertry means for recharging the second electric power supply source, when it has been discharged; opening the third switching means after the second electric power supply source has been charged for applying a second DC voltage obtained from the second electric power supply source to the first terminal of the bidirectional voltage converting means for performing a step-up voltage converting operation and supplying the predetermined DC voltage to the output means; and closing the first switching means, while opening the second switching means and opening the third switching means, for stepping up the first DC voltage via the bidirectional voltage converting means and supplying the predetermined DC voltage to the output means. a bidirectional voltage converting means, which includes a first terminal and a second terminal, for performing step-up converting or step-down converting between the first terminal and the second terminal; a first switching means for connecting the first electric power supply source (140; 150) to the first terminal of the bidirectional voltage converting means; a second switching means for connecting the second electric power supply source to the first terminal of the bidirectional voltage converting means; a third switching means for connecting the first electric power supply source (140; 150) to the second terminal of the bidirectional voltage converting means; an output means for outputting a DC voltage at the second terminal of the bidirectional voltage converting means as the predetermined DC voltage; and a switching control means for controlling switching of the first switching means, the second switching means and the third switching means, wherein the switching control means is capably of: opening the first switching means, while closing the second switching means and closing the third switching means, for applying a first DC voltage obtained from the first electric power supply source (140; 150) to the second terminal of the bidirectional voltage converting means to transmit electric power to the second electric power supply source via the bidirectional voltage convertry means for recharging the second electric power supply source, when it has been discharged; opening the third switching means after the second electric power supply source has been charged for applying a second DC voltage obtained from the second electric power supply source to the first terminal of the bidirectional voltage converting means for performing a step-up voltage converting operation and supplying the predetermined DC voltage to the output means; and closing the first switching means, while opening the second switching means and opening the third switching means, for stepping up the first DC voltage via the bidirectional voltage converting means and supplying the predetermined DC voltage to the output means. opening the first switching means, while closing the second switching means and closing the third switching means, for applying a first DC voltage obtained from the first electric power supply source (140; 150) to the second terminal of the bidirectional voltage converting means to transmit electric power to the second electric power supply source via the bidirectional voltage convertry means for recharging the second electric power supply source, when it has been discharged; opening the third switching means after the second electric power supply source has been charged for applying a second DC voltage obtained from the second electric power supply source to the first terminal of the bidirectional voltage converting means for performing a step-up voltage converting operation and supplying the predetermined DC voltage to the output means; and closing the first switching means, while opening the second switching means and opening the third switching means, for stepping up the first DC voltage via the bidirectional voltage converting means and supplying the predetermined DC voltage to the output means.