Provided is a wind turbine generator in which, the specifications of the gears can be designed for the level of a maximum load during driving without decreasing the necessary strength and durability. A wind turbine generator in which a nacelle is installed at an upper end of a tower that is vertically erected on a foundation includes a yaw driving unit that yaws the nacelle and/or a pitch driving unit that controls pitch angles of the wind turbine blades, wherein a drive mechanism of the yaw driving unit and/or the pitch driving unit, which are equipped with an engaging portion of a slewing bearing (20) having internal teeth or external teeth and a pinion gear is equipped with a clutch mechanism, at the engaging portion, that slides when receiving an input larger than a maximum load during driving.