Internal combustion engine silencer

A muffler for internal combustion engines, in order to achieve a high attenuating effect without reducing the attenuating effect in the high revolution range of the engine, comprising one or more expanding chambers 11 through 13 formed in a muffler 1 , an inlet pipe 5 which opens into a expanding chamber 11 and introduces exhaust gas into the muffler 1 , and an outlet pipe 6 which opens into other expanding chamber 13 and introduces exhaust gas out of the muffler 1 . In the muffler, the exhaust gas discharged through an aperture 5 a of this inlet pipe 5 is inverted once or more times in the muffler 1 and is introduced into an aperture 6 a of the outlet pipe 6 . In a contraflow area L 2 in which the flowing direction C of exhaust gas flowing in the expanding chambers 11 through 13 and the flowing direction E of exhaust gas flowing in the outlet pipe 6 are not the same, interference holes 7 composing of a plurality of small holes communicating with the expanding chambers 11 through 13 are bored in the outlet pipe 6 . The aperture area of the small holes 7 is kept at not less than 1% but not more than 5% of the surface area of the outlet pipe in the contraflow area L 2.