PURPOSE: To make the output of a power geneator constant by providing a vertically operated windmill and rotating it at constant velocity according to a wind velocity.

CONSTITUTION: Generally a wind velocity is increased proportional to almost the cube of a height from the ground. Therefore, a wind velocity to a windmill 1 can be maintained constantly by lowering the windmill 1 if a wind velocity is increased and lifting the windmill if a wind velocity is reduced. A prearranged wind velocity for a power generator to obtain a constant output is beforehand connected to a control panel inside an operating chamber 13 and the windmill 1 is vertically operated through the intermediary of a lifting device 11 at "ascending" command or "descending" command meeting the output value of a wind velocity detector 7. In this manner, the output of a power generator is set constant and it is not stopped even at time of a strong wind. In addition, it is possible to carry out repairing or ajustment of the windmill at a position near the ground by lowering the lifting device.