A wind turbine generator comprising: a hub; a main shaft coupled to the hub; a generator which converts rotation energy transmitted from the main shaft to electric power; a hydraulic pump of variable displacement type which is rotated by the main shaft; a hydraulic motor of variable displacement type which is connected to the generator; a high pressure oil line which is arranged between a discharge side of the hydraulic pump and an intake side of the hydraulic motor; a low pressure oil line which is arranged between an intake side of the hydraulic pump and a discharge side of the hydraulic motor; and a control unit which has a pump controller and a motor controller, the pump controller adjusting a displacement D p of the hydraulic pump, the motor controller adjusting a displacement D m of the hydraulic motor; characterised in that the wind turbine generator further comprises an operation mode selection unit which switches an operation mode between a normal operation mode and a low rotation speed operation mode, wherein the control unit controls the pump controller and the motor controller such that the rotation speed of the main shaft is variable depending on the wind speed when the normal operation mode is selected by the operation mode selection unit and the wind speed falls within a range more than a cut-in wind speed and less than a first rated wind speed, wherein the control unit controls the pump controller and the motor controller such that the rotation speed of the main shaft is set constant at a first rated rotation speed when the normal operation mode is selected by the operation mode selection unit and the wind speed is equal to or more than the first rated wind speed, wherein the control unit controls the pump controller and the motor controller such that the rotation speed of the main shaft is variable depending on the wind speed when the low rotation speed operation mode is selected by the operation mode selection unit and the wind speed falls within a range more than the cut-in wind speed and less than a second rated wind speed, and wherein the control unit controls the pump controller and the motor controller such that at least one of a rated displacement of the hydraulic pump and a rated pressure of the high pressure oil line are greater than that of the normal operation mode, the rotation speed of the main shaft is set constant at the second rated rotation speed which is lower than the first rated rotation speed, and the rated output of electric power in the low rotation speed operation mode is substantially the same as that of the normal operation mode, when the low rotation speed operation mode is selected by the operation mode selection unit and the wind speed is equal to or more than the second rated wind speed. a hub; a main shaft coupled to the hub; a generator which converts rotation energy transmitted from the main shaft to electric power; a hydraulic pump of variable displacement type which is rotated by the main shaft; a hydraulic motor of variable displacement type which is connected to the generator; a high pressure oil line which is arranged between a discharge side of the hydraulic pump and an intake side of the hydraulic motor; a low pressure oil line which is arranged between an intake side of the hydraulic pump and a discharge side of the hydraulic motor; and a control unit which has a pump controller and a motor controller, the pump controller adjusting a displacement D p of the hydraulic pump, the motor controller adjusting a displacement D m of the hydraulic motor; characterised in that the wind turbine generator further comprises an operation mode selection unit which switches an operation mode between a normal operation mode and a low rotation speed operation mode, wherein the control unit controls the pump controller and the motor controller such that the rotation speed of the main shaft is variable depending on the wind speed when the normal operation mode is selected by the operation mode selection unit and the wind speed falls within a range more than a cut-in wind speed and less than a first rated wind speed, wherein the control unit controls the pump controller and the motor controller such that the rotation speed of the main shaft is set constant at a first rated rotation speed when the normal operation mode is selected by the operation mode selection unit and the wind speed is equal to or more than the first rated wind speed, wherein the control unit controls the pump controller and the motor controller such that the rotation speed of the main shaft is variable depending on the wind speed when the low rotation speed operation mode is selected by the operation mode selection unit and the wind speed falls within a range more than the cut-in wind speed and less than a second rated wind speed, and wherein the control unit controls the pump controller and the motor controller such that at least one of a rated displacement of the hydraulic pump and a rated pressure of the high pressure oil line are greater than that of the normal operation mode, the rotation speed of the main shaft is set constant at the second rated rotation speed which is lower than the first rated rotation speed, and the rated output of electric power in the low rotation speed operation mode is substantially the same as that of the normal operation mode, when the low rotation speed operation mode is selected by the operation mode selection unit and the wind speed is equal to or more than the second rated wind speed.