To provide a wind mill as a power source to be efficiently rotated by wind in a range from a weak wind to a strong wind by adjusting resistance to be applied to the wind mill in response to the wind power.

Variable blades 2 are varied in response to the wind power applied to the variable blades 2 in a horizontal shaft provided inside a vertical shaft wind mill to increase/decrease resistance against the wind, and the vertical shaft wind mill 1 is thereby efficiently rotated with the wind in a range from a weak wind to a strong wind.