Abstract: In the wind turbine generator, a front wind rotor includes a blade designed, in an area outer than an intermediate of a rotation radius of the front wind rotor, to have a blade element comprised of an aerofoil being cambered in a cross-section along a rotational direction thereof, and is radially twisted so as to ensure an angle of attack smaller by a predetermined stable margin angle than an angle of attack providing a maximum lift-drag ratio regardless of a radial position, and further designed, in an area inner than the intermediate of a rotation radius of the front wind rotor, to be radially twisted so as to ensure an angle of attack at which a drag acting on the blade element is made small, and a total force of the drag and a lift force is axially directed, regardless of a radial position.