A wind turbine generator includes a tower, at least one blade, a hub supporting the blade, a nacelle supported by the tower and having a duct part with an intake port and an exhaust port, and a heat exchanger provided in the duct part and cooling a cooling medium for a heat-producing component inside the tower or nacelle. The wall of the nacelle has a double wall structure constituted of an inner wall and an outer wall in an area where the duct part is provided. The inner wall forms a bottom surface of the duct part and has a curved portion which curves inward toward a center line of the nacelle with increasing distance from the hub. The duct part increases in cross-section from a side of the intake port to a side of the exhaust port at least in an area where the curved portion is formed.