An object of the present invention is to provide a wind turbine generator and an operation control method thereof, which takes measures against issues such as noise, bird strike and a friction heat of a bearing shaft. The wind turbine generator 1 comprises a hydraulic pump 12 of a variable displacement type which is rotated by the main shaft 8, a hydraulic motor 14 of a variable displacement type which is connected to the generator 20, and a high pressure oil line 16 and a low pressure oil line 18 which are arranged between the hydraulic pump 12 and the hydraulic motor 14. An operation mode selection unit 38 switches an operation mode between a normal operation mode and a low rotation speed operation mode. The low rotation speed operation mode has a rated rotation speed of the main shaft 8 lower than that of the normal operation mode, at least one of a rated displacement of the hydraulic pump 12 and a rated pressure of the high pressure oil line that are greater than that of the normal operation mode and a rated output of a generator 20 substantially same as that of the normal operation mode. The operation mode selection unit 38 selects the normal operation mode or the low rotation speed operation mode depending on environment conditions.