To provide an auxiliary power supply device for using an external storage device outdoors, where any commercial power source does not exist, that has been conventionally unusable outdoors with the power(for example, 5V of USB specifications) of an interface outputted by information processor terminal equipment.

This external storage device is provided with an interface 6 for power input and signal input/output connected through a first power/signal cable 5 to an interface 4 for external storage device connection of information processing terminal equipment 1 such as a personal computer. The auxiliary power supply device 3 is built in with a battery 18, a charging circuit 7 for charging a power supplied through the interface 6 for power input and signal input/output to the battery 18 and a converting circuit 8 for converting the voltage or/and currents of the charged power. This external storage device is also provided with an interface 11 for converted power output and signal input/output connected through a second power/signal cable 10 to an interface 9 for information processing terminal connection of the external storage device 2. The converted output outputted from the converting circuit 8 is supplied to the external storage device 2 so that the external storage device can be operated.