engine room 13 and enters the muffler room 15 after flowing along with outer wall of the generator body 17 and the engine 16 since boundary between the engine room 13 and the muffler room 15 is separated by the partition plate 25 and outer wall 22 a of the muffler 22 the cooling air entering the muffler room 15 is once guided downwardly along with periphery of the muffler 22 as shown by arrow w 3 after that the cooling air goes up along with periphery of the muffler 22 as shown by arrows w 4 w 5 passes through the outlet 26 of the cooling air and is discharged into the exhaust duct 9 the cooling air discharged into the exhaust duct 9 hits the interior surface of lower wall surface 9 b of the exhaust duct 9 and its direction is changed along with the straightening vane 27 located at upper part thereof after that the cooling air hits upper interior surface of the exhaust duct 9 its direction is further changed and is discharged outdoor after passing through the duct outlet 9 a in other words shapes and arrangements of wall surface of the exhaust duct 9 and the straightening vane 27 are set so that the cooling.