PURPOSE: To smoothen the starting, by supplying no field current to a field winding until a rotational speed of a generator reaches a power generation starting rotational speed.

CONSTITUTION: A torque of a windmill 1 is transmitted to a generator 2. A rotational speed detector 5 feeds an output to a field regulator 6, which regulates a field winding 21 so as to control an output current from the generator 2. A battery 8 provides an energy when a rotational speed of the generator 2 has reached a power generation starting rotational speed, but it does not supply a current to the field winding 21 until the rotational speed of the generator 2 reaches the power generation starting rotational speed. Accordingly, the load of the windmill 1 may be reduced to thereby permit smooth starting of the windmill and smooth starting of the power generation.