A wind turbine generator comprising: a nacelle; a device placed in the nacelle; a cooling duct comprising a duct inlet and a duct outlet which are provided in a wall surface of the nacelle, the cooling duct forming a duct interior space between the duct inlet and the duct outlet independently of an interior space of the nacelle; a rotor configured to rotate upon receiving wind; a generator placed in the nacelle to convert rotational energy of the rotor into electric power; and a main shaft disposed in the nacelle and connected to the rotor; wherein the cooling duct is configured to lead ambient air introduced directly from the duct inlet without a filter to a cooling object part of the device disposed in the duct interior space, wherein the duct outlet is disposed on a downwind side of the duct inlet, wherein an inlet port for ventilating the interior space of the nacelle is provided on an upwind side of the duct outlet to draw ambient air for cooling to the interior space of the nacelle, the wind turbine generator being characterized in that: the duct inlet is disposed at a lower position than the cooling object part, the cooling object part includes a heat radiating part of the generator, the duct inlet and the duct outlet are provided on a side surface of the nacelle among the wall surfaces (3A, 3B, 3C, 3D, 3E) of the nacelle, the inlet port is provided on said side surface of the nacelle the generator is disposed at a side of the main shaft between the main shaft and said side surface of the nacelle. a nacelle; a device placed in the nacelle; a cooling duct comprising a duct inlet and a duct outlet which are provided in a wall surface of the nacelle, the cooling duct forming a duct interior space between the duct inlet and the duct outlet independently of an interior space of the nacelle; a rotor configured to rotate upon receiving wind; a generator placed in the nacelle to convert rotational energy of the rotor into electric power; and a main shaft disposed in the nacelle and connected to the rotor; wherein the cooling duct is configured to lead ambient air introduced directly from the duct inlet without a filter to a cooling object part of the device disposed in the duct interior space, wherein the duct outlet is disposed on a downwind side of the duct inlet, wherein an inlet port for ventilating the interior space of the nacelle is provided on an upwind side of the duct outlet to draw ambient air for cooling to the interior space of the nacelle, the wind turbine generator being characterized in that: the duct inlet is disposed at a lower position than the cooling object part, the cooling object part includes a heat radiating part of the generator, the duct inlet and the duct outlet are provided on a side surface of the nacelle among the wall surfaces (3A, 3B, 3C, 3D, 3E) of the nacelle, the inlet port is provided on said side surface of the nacelle the generator is disposed at a side of the main shaft between the main shaft and said side surface of the nacelle. the duct inlet is disposed at a lower position than the cooling object part, the cooling object part includes a heat radiating part of the generator, the duct inlet and the duct outlet are provided on a side surface of the nacelle among the wall surfaces (3A, 3B, 3C, 3D, 3E) of the nacelle, the inlet port is provided on said side surface of the nacelle the generator is disposed at a side of the main shaft between the main shaft and said side surface of the nacelle.