Abstract: A semi-active muffler for an internal combustion engine comprising a case defining a closed space with a predetermined capacity, the case being partitioned into first, second, and third chambers, a first bypass pipe for communicating the first chamber with the third chamber, a second bypass pipe for communicating the second chamber with the third chamber, an inlet pipe for introducing exhaust gas into the first and second chambers, an outlet pipe for exhausting the exhaust gas introduced into the third chamber from the first and second chambers respectively through the first and second bypass pipes, a valve for selectively opening a downstream end of the second bypass pipe according to a pressure level within the second chamber, and buffer means for reducing an impact and noise when the valve is opened and closed, wherein the buffer means comprises a gap kept between the valve and the downstream end of the second bypass pipe even when the valve is in a completely closed position.