PURPOSE: To improve the energy conversion efficiency of the wind mill by applying the sail wing to the vertical shaft type wind mill.

CONSTITUTION: When feeding the wind to the vertical shaft type wind mill, the front edge board 5 supported rotatably against L-shaped arm 3 will rotate in accordance to the direction of the wind thereby the sail wing will change the angle automatically to maintain the proper direction against the combined speed of the wind and the wind mill. Furthermore the angle of attack of the sail wing will be varied automatically by the flexure of the stretched wire at the rear edge section of the wing or the expansion of the spring 8 fixed to it, thereby the flexible film 6 will flex to form the wing shape suitable for the flow. Consequently, the sail wing will rotate efficiently around the vertical axis 1.