To make a portable device usable even while its auxiliary battery is charged by selecting a commercial power supply or a built-in power supply of the auxiliary battery for output, while monitoring power supply input condition to the auxiliary battery.

The auxiliary battery is constituted of a power supply input part (1) inputting power from outside, an electricity storage part (2) provided with a secondary battery (21) to which surplus power from the power supply input part is supplied, and a power supply output part (3) to which a power is supplied either from the electricity storage part or the power supply input part, which is provided with two or more of the external outputs corresponding to external power supply specifications, and externally outputs different voltages simultaneously by an output control circuit (31) provided with an input changeover circuit (30) which supplies the output power from the power supply input part or the discharging power from the secondary battery. In the secondary battery a protection circuit (22) which stops charging/discharging and/or shuts off circuit and an indication means (23) which indicates the remaining capacity are provided.