A wind power generator system comprising: a windmill rotor including a blade whose pitch angle is variable; a generator driven by said windmill rotor; and a control unit generating a real power command to control output power of said generator and controlling said pitch angle of said blade in response to a rotational speed of said windmill rotor or said generator, wherein said control unit performs a first control in which said real power command is generated in accordance with a predetermined power-rotational speed curve until said rotational speed is increased to reach a predetermined rated rotational speed, and performs a second control in which said real power command is kept at a predetermined rated power when said rotational speed exceeds said rated rotational speed, wherein said control unit is responsive to said pitch angle for maintaining a state of performing said second control or for switching to a state of performing said first control, when said rotational speed is reduced below said rated rotational speed after said control unit is once set to the state of performing said second control, and wherein, when said rotational speed is reduced below said rated rotational speed after said control unit is once set to the state of performing said second control, if said rotational speed is not less than an intermediate rotational speed, said control unit maintains the state of performing said second control for a case when said pitch angle is larger than a predetermined pitch angle, and does not switch to the state of performing said first control until said pitch angle reaches said predetermined pitch angle, and if said rotational speed is less than the intermediate rotational speed, said control unit switches from the state of performing said second control to the state performing said first control irrespective of the pitch angle. a windmill rotor including a blade whose pitch angle is variable; a generator driven by said windmill rotor; and a control unit generating a real power command to control output power of said generator and controlling said pitch angle of said blade in response to a rotational speed of said windmill rotor or said generator, wherein said control unit performs a first control in which said real power command is generated in accordance with a predetermined power-rotational speed curve until said rotational speed is increased to reach a predetermined rated rotational speed, and performs a second control in which said real power command is kept at a predetermined rated power when said rotational speed exceeds said rated rotational speed, wherein said control unit is responsive to said pitch angle for maintaining a state of performing said second control or for switching to a state of performing said first control, when said rotational speed is reduced below said rated rotational speed after said control unit is once set to the state of performing said second control, and wherein, when said rotational speed is reduced below said rated rotational speed after said control unit is once set to the state of performing said second control, if said rotational speed is not less than an intermediate rotational speed, said control unit maintains the state of performing said second control for a case when said pitch angle is larger than a predetermined pitch angle, and does not switch to the state of performing said first control until said pitch angle reaches said predetermined pitch angle, and if said rotational speed is less than the intermediate rotational speed, said control unit switches from the state of performing said second control to the state performing said first control irrespective of the pitch angle.