A power supply device includes battery interfaces configured to removably receive at least three batteries and a connection circuit configured to electrically connect the at least three batteries to each other. The connection circuit is capable of connecting at least two batteries in parallel and connecting at least one other battery to the at least two parallel-connected batteries in series. According to this power supply device, it is possible to supply high power to an electric device by connecting the batteries in series. In addition, it is possible to supply almost all of the electric power stored in the batteries to the electric device, even if amount of the remaining electric power in each battery is substantially uneven.