To provide a vertical shaft windmill having flexibility and high efficiency, for enlarging the utilization of the wind energy.

A blade 3 having a rotary shaft 4, is arranged on a rotor 2 having a vertical shaft 1. A ratio of the rotation of the rotor and that of the blade are set by a timing belt 5 and a pulley 6, so that the blade makes a half-rotation during one rotation of the rotor. A direction of the blade is a radius direction at first, then it is located on a position perpendicular to a flow line of the wind, then the torque is effectively generated at each angle of rotation when the rotor is successively rotated, and the minus torque of the reverse rotation becomes zero or minimum, whereby the windmill is rotated.