The object of the present invention is to provide a wind power generation system characterized by simplified field weakening of the magnetic flux of the permanent magnet of a shaft generator and a speed change control over an extensive range. In a wind power generation system comprising a main shaft where a vane is mounted, a generator connected to the main shaft for transmitting the rotating power of the vane, an inverter for converting the electric power of the generator, a controller for controlling the inverter, a means for controlling the pitch of the vane meeting the wind velocity requirements, a brake, and an anemometer/anemoscope; the present invention is characterized in that the rotor of the rotary machine comprising a permanent magnet is split to allow relative movement.