When a wind velocity sensor detects a wind velocity higher than a predetermined level, a timer device starts a waiting timer function during an operation timer period shorter than the waiting timer period. After the waiting timer period is over, the waiting timer function is switched to an operation timer function, and a switching relay switches a generator to a motor only during an operation timer period. Then, a driving circuit performs a start assisting rotation. After the operation timer period is over, the waiting timer period re-starts. This process is repeated. When during this period, the number of rotations of a rotor monitored by a rotation number measuring device, based on the output voltage Vm of a three-phase conduction coil exceeds a predetermined number of rotations, the charging of a battery by a three-phase generator is started.