A power supply device including vehicle connecting portions, a load connecting portion, a power supply portion, a memory portion for storing the number of charges and discharges for each storage-battery-equipped vehicle, and a supply control portion for performing control such that the power supply portion performs a discharge operation only on a selected supply vehicle. The supply control portion selects as the supply vehicle one of the storage-battery-equipped vehicles that satisfies a first condition that an SOC of a storage battery is greater than or equal to a prescribed value and a second condition that the number of charges and discharges is not the highest among the storage-battery-equipped vehicles.