A wind turbine generator includes a hydraulic pump rotatable by a main shaft, a hydraulic motor connected to a generator, and a high pressure oil line and a low pressure oil line which are arranged between a hydraulic pump and a hydraulic motor. An operation mode selection unit switches an operation mode between a normal operation mode and a low rotation speed operation mode depending on environment conditions. The low rotation speed operation mode has a rated rotation speed of the main shaft lower than that of the normal operation mode, at least one of a rated displacement of the hydraulic pump 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 substantially same as that of the normal operation mode.
