PURPOSE: To make improvements in output control over a Darrieus type windmill system in terms of responsiveness as well as to aim at the promotion of expeditiousness as compared with stall control and variable pitch control so far used, by changing the turning radius of a vane with a driving means and controlling a generating output so as to be maintained to the specified value.

CONSTITUTION: When a vane 1 receives wind energy, this lift is converted into energy rotating around a 13 via a column 2 and a rotor 12. Incidentally, if a wind varies to a high speed, rotation of the vane 1 increases, and thereby the output of a generator 17 goes up. At this time, it is operated by a controller 14, and a motor 8 is controlled so as to make the output of the generator 1 7 become the specified value. In brief, a sleeve shaft 4 is made to go up by the motor 8 via a rack 9a, thereby increasing a lift value 1 to some extent. In consequence, since the column 2 shifts via a link rod 3 and a tuning radius of the vane 1 is decreased, the output of the generator 17 is maintained to the set value. Therefore output control over a Darrieus type windmill system is expeditiously performable so well in terms of responsiveness.