A wind turbine generator system comprising: a wind turbine rotor including a blade whose pitch angle (β) is variable; a generator driven by said wind turbine rotor; and a control unit controlling output power of said generator and said pitch angle (β) of said blade in response to a rotational speed (ω) of said wind turbine rotor or said generator, wherein said control unit performs a first control in which said output power is controlled in accordance with a predetermined power-rotational speed curve until said rotational speed (ω) is increased to reach a predetermined rated rotational speed (ω max), and performs a second control in which said output power is controlled to a predetermined rated power (P rated) when said rotational speed (ω) exceeds said rated rotational speed (ω max), and wherein, when said rotational speed (ω) is reduced below said rated rotational speed (ω max) after said control unit is once set to the state of performing said second control, said control unit maintains the state of performing said second control for a case when said pitch angle (β) is larger than a predetermined pitch angle (β min), and does not switch to the state of performing said first control until said pitch angle (β) reaches said predetermined pitch angle (β min), and said control unit is switched to the state of performing said first control irrespectively of said pitch angle (β), when said rotational speed (ω) is reduced below a predetermined threshold rotation speed (ω ' M) which is lower than said rated rotational speed (ω max) and which is greater than an intermediate rotational speed (ω M), that is an average of the rated rotational speed (ω max) and a minimum rotational speed (ω min), after said control unit is once placed into the state of performing said second control. a wind turbine rotor including a blade whose pitch angle (β) is variable; a generator driven by said wind turbine rotor; and a control unit controlling output power of said generator and said pitch angle (β) of said blade in response to a rotational speed (ω) of said wind turbine rotor or said generator, wherein said control unit performs a first control in which said output power is controlled in accordance with a predetermined power-rotational speed curve until said rotational speed (ω) is increased to reach a predetermined rated rotational speed (ω max), and performs a second control in which said output power is controlled to a predetermined rated power (P rated) when said rotational speed (ω) exceeds said rated rotational speed (ω max), and wherein, when said rotational speed (ω) is reduced below said rated rotational speed (ω max) after said control unit is once set to the state of performing said second control, said control unit maintains the state of performing said second control for a case when said pitch angle (β) is larger than a predetermined pitch angle (β min), and does not switch to the state of performing said first control until said pitch angle (β) reaches said predetermined pitch angle (β min), and said control unit is switched to the state of performing said first control irrespectively of said pitch angle (β), when said rotational speed (ω) is reduced below a predetermined threshold rotation speed (ω ' M) which is lower than said rated rotational speed (ω max) and which is greater than an intermediate rotational speed (ω M), that is an average of the rated rotational speed (ω max) and a minimum rotational speed (ω min), after said control unit is once placed into the state of performing said second control.