PURPOSE: To obtain the minimum value of chaniging amount of the pitch of a blade to control the rotating number of the blade efficiently and improve the converting efficiency of wind energy by a method wherein the decision result of increasing rate of the rotating number is utilized in addition to the decision result of comparison of the rotating number of the blade with the rated value thereof.

CONSTITUTION: The device is provided with a rotating number decision means 102, deciding the rotating number by comparing it with a set value based on the rotating number read by a rotatig number detecting means 101 detecting the output of a tachometer 6, and a rotating number increasing rate decision means 103, operating and deciding the increasing rate of rotating number from this rotating number and the same detected previously. In order to drive the blade of variable pitch device 4 into the direction to restrict the rotating number based on the decision results of said both means 102, 103, ON-OFF signals are outputted by a blade pitch changing means 104 and the variable pitch device 4 is operated in accordance with these signals. According to this method, the minimum value of the changing amount of pitch of the blade may be obtained and the reduction of the rotating number after the control may be minimized.