Abstract: A muffler for an internal combustion engine, capable of sufficiently attenuating exhaust sound and preventing power output from being reduced by allowing exhaust gas to flow without a hitch and generating vortices. The muffler comprises a casing, a vortex diffusing member, a vortex generating cylinder, and a vortex generating member. The vortex diffusing member is mounted in the casing and is connected to an exhaust inlet pipe. The vortex generating cylinder is positioned in the casing at its first side and is protruded into the exterior at its second side, and is provided with a tail pipe at its exterior end. The vortex generating member is axially mounted in the vortex generating cylinder.