PURPOSE: To prevent a windmill from being damaged by setting up wind-guide plates higher than the windmill radially round the windmill and arranging shutter plates outside between the wind-guide plates to be opened and closed for collecting wind efficiently at soft wind and regulating the wind power at strong wind.

CONSTITUTION: A windmill 1 is connected to a generator via a shaft 8. Wind- guide plates 11 are set up radially round the windmill 1. A bore 13 is bored to be used for a draft hole 14 at the center of each wind-guide plate 11. Outside each wind channel 15 between these wind-guide plates 11, a shutter plate 12 is provided to be rotatable round a shaft 16. A sprocket fast fixed on the end of each shaft 16 is successively connected by a chain 17, and a motor 20 is provided for driving the chain. The motor 20 is driven on the basis of the detection signal of a wind-power detection sensor 10, and shutter plates 12 are controlled to keep the optimum opening in accordance with the strength of wind.