A wind turbine generator comprising: a rotor configured to rotate upon receiving wind; a generator for converting rotational energy of the rotor to electric power; a circuit breaker for changing a connection state of the generator to a grid; characterized in that: the wind turbine generator further comprises: a circuit breaker controller for controlling opening and closing of the circuit breaker when output of the generator becomes less than a lower limit based on a prediction result of whether or not the output of the generator returns to a value at or above the lower limit in a time frame from a present point to a future point; and a prediction unit for predicting whether or not the output of the generator returns to a value at or above the lower limit in the time frame based on at least one of a future wind speed impinging on the rotor,, the circuit breaker controller is configured to: maintain such a state that the circuit breaker is closed and the generator is connected to the grid, when it is predicted that the output returns to a value at or above the lower limit in the time frame; disconnect the generator from the grid by opening the circuit breaker, when it is predicted that the output continues to remain less than the lower limit in the time frame; and control opening and closing of the circuit breaker based on a prediction result by the prediction unit. a rotor configured to rotate upon receiving wind; a generator for converting rotational energy of the rotor to electric power; a circuit breaker for changing a connection state of the generator to a grid; characterized in that: the wind turbine generator further comprises: a circuit breaker controller for controlling opening and closing of the circuit breaker when output of the generator becomes less than a lower limit based on a prediction result of whether or not the output of the generator returns to a value at or above the lower limit in a time frame from a present point to a future point; and a prediction unit for predicting whether or not the output of the generator returns to a value at or above the lower limit in the time frame based on at least one of a future wind speed impinging on the rotor,, the circuit breaker controller is configured to: maintain such a state that the circuit breaker is closed and the generator is connected to the grid, when it is predicted that the output returns to a value at or above the lower limit in the time frame; disconnect the generator from the grid by opening the circuit breaker, when it is predicted that the output continues to remain less than the lower limit in the time frame; and control opening and closing of the circuit breaker based on a prediction result by the prediction unit. the wind turbine generator further comprises: a circuit breaker controller for controlling opening and closing of the circuit breaker when output of the generator becomes less than a lower limit based on a prediction result of whether or not the output of the generator returns to a value at or above the lower limit in a time frame from a present point to a future point; and a prediction unit for predicting whether or not the output of the generator returns to a value at or above the lower limit in the time frame based on at least one of a future wind speed impinging on the rotor,, a circuit breaker controller for controlling opening and closing of the circuit breaker when output of the generator becomes less than a lower limit based on a prediction result of whether or not the output of the generator returns to a value at or above the lower limit in a time frame from a present point to a future point; and a prediction unit for predicting whether or not the output of the generator returns to a value at or above the lower limit in the time frame based on at least one of a future wind speed impinging on the rotor,, the circuit breaker controller is configured to: maintain such a state that the circuit breaker is closed and the generator is connected to the grid, when it is predicted that the output returns to a value at or above the lower limit in the time frame; disconnect the generator from the grid by opening the circuit breaker, when it is predicted that the output continues to remain less than the lower limit in the time frame; and maintain such a state that the circuit breaker is closed and the generator is connected to the grid, when it is predicted that the output returns to a value at or above the lower limit in the time frame; disconnect the generator from the grid by opening the circuit breaker, when it is predicted that the output continues to remain less than the lower limit in the time frame; and control opening and closing of the circuit breaker based on a prediction result by the prediction unit.