Upon receiving a supply start instruction, each vehicle (2, 3, 4) transmits its own identification ID to other vehicles (sequence SQ12, SQ14, SQ16). A vehicle (2) determining that it is a master itself performs notification of master to the vehicles (3, 4) (sequence SQ20) and begins generation of an AC voltage conforming to its own period (sequence SQ22). Each vehicle (3, 4) generates an AC current synchronized with a voltage reference which is the AC voltage generated from the vehicle (2) (sequence SQ26a, SQ26b). In this way, the vehicles (2, 3, 4) cooperate to begin power supply to a power load.