A battery and a DC-to-DC converter are contained in a battery pack. When the battery pack is connected to a load by means of a connector and an output voltage reference value, corresponding to the power supply voltage of the load, the output voltage of the battery is converted according to the output voltage reference value by the DC-to-DC converter. The output voltage reference value is set by an output voltage reference value output means in the connector. Furthermore, the converted voltage is supplied to the load via an output voltage line.