An object of the present invention is to provide a wind turbine generator system and an operation control method thereof, which has a superior operation efficiency and stability at a low wind speed and during the occurrence of a gust and is equipped with Ride Through function at Grid low voltage condition. The wind turbine generator comprises a hydraulic pump of a variable displacement type which is rotated by the main shaft, a hydraulic motor of a variable displacement type which is connected to the generator, and a high pressure oil line and a low pressure oil line which are arranged between the hydraulic pump and the hydraulic motor. An accumulator is connected to the high pressure oil line via an accumulator valve. ACC valve control unit controls opening and closing of the accumulator valve based on at least one of wind speed and the state of the grid.