Provided is a wind turbine generator system that can cool a nacelle interior by ventilation irrespective of the outside air temperature in various installation environments. In a wind turbine generator system in which equipment of drive and electricity generation mechanisms that are coupled to a rotor head provided with wind turbine blades is disposed inside a nacelle, and in which, by driving a ventilation fan provided in the nacelle, outside air introduced from an air inlet provided in a front face of the nacelle is exhausted outside the nacelle from an air outlet that communicates with a fan outlet, to perform ventilation cooling of the interior, a side air inlet is additionally provided in a side face of the nacelle at a position where a negative pressure is generated by air flowing outside the nacelle.