PURPOSE: To permit the safe operation of a windmill even at the strong or weak wind speed by adjusting the amount of wind introduced into the windmill by installing the dampers into a wind gathering box and by controlling the dampers, according to the wind speed.

CONSTITUTION: A part of the side wall of a wind gathering box 1 is constituted of a plurality of inside dampers 2 and outside dampers 22. The inside damper 2 is opened and closed inwardly to discharge the wind amount in a wind gathering box 1. While, the outside damper 22 is opened and closed outward to adjust the amount of wind entering into the wind gathering box 1. The dampers 2 and 22 are controlled by a control motor 18 through the detection of the wind speed. The revolution of the control motor 18 is transmitted to the lead screws 8 and 8 for the dampers by a sprocket 19 and a belt 20. Further, the revolution is transmitted as reciprocating movement into the dampers 2 and 22 through the rods 5 and 5 and damper supporting metal fittings 3 and 3. Therefore, each opening degree of the dampers 2 and 22 can be controlled.