A wind turbine generator including a nacelle having reduced size and weight is provided. The wind turbine generator includes a nacelle disposed on a tower. The nacelle includes a main shaft that is connected to a rotor head equipped with blades and that integrally rotates with the rotor head, a gearbox that increases the rotational speed of the main shaft and that outputs the resulting rotational speed, and a generator driven by the output from the gearbox. In the wind turbine generator, a drivetrain extending from the main shaft to the generator via the gearbox is disposed in the rotor head.