Reduce noises emitted from inlet and outlet of cooling air of a soundproof enclosed type generator. An inlet 20 and an outlet 26 of the cooling air are provided to the front and rear of a chassis 2, respectively. An air supply duct 8 is detachably mounted to the inlet 20, and an exhaust duct 9 is detachably mounted to the outlet 26. An air taking-in opening 8a facing downward is provided to the air supply duct 8 which guides the air towards horizontally opened inlet 20. The exhaust duct 9 is equipped with wall surface and a straightening vane 27 at a position opposed to the cooling air discharged from the outlet 26 to change the cooling air flow in complicated fashion to discharge it outside. Sound-absorption materials are pasted to the interior surface of the air supply duct 8 and the exhaust duct 9.