The present invention provides a wind turbine generator in which an oil seal structure provided in a blade rotating ring bearing of a wind turbine blade is improved and leakage of lubricant oil to outside is prevented or suppressed. A wind turbine generator in which a rotor head has wind turbine blades, the rotor head rotates when the wind turbine blades receive wind power and drives a power generator disposed in a nacelle to generate electricity, the nacelle is disposed on an upper end of a tower that stands on a foundation such that the nacelle can yaw-slew, a blade rotating ring bearing is disposed between the wind turbine blade and the rotor, the wind turbine generator includes a pitch control mechanism that can adjust a pitch angle of the wind turbine blades, wherein seal mechanisms are mounted on both ends of opposed rolling body holding surfaces of an outer race and an inner race of the blade rotating ring bearing with rolling bodies interposed between both the ends, sides of the seal mechanisms closer to the blade are formed into a double sealed structure in which a space is disposed between seal members and the seal member are disposed in an axial direction of the blade, pressurized fluid is supplied to the space, and lips of the seal members are pushed against a mating sidewall surface.