In a power supply system using various kinds of storage batteries such as a large capacity storage battery or a small capacity storage battery, an electric power path is complicated, which may increase failures such as a short circuit trouble. There is a problem in that, when a short circuit trouble between storage batteries occurs, damage to a smaller capacity storage battery is serious. Accordingly, in a power supply system according to the present invention, the relationship among voltages, current capacities, and electric power capacities of various kinds of storage batteries such as a large capacity storage battery or a small capacity storage battery is defined so that damage to the storage batteries may be minimized even when malfunction occurs in, for example, switching a switch, and energy transfers from a higher voltage storage device.