A catalyzer-containing muffler for treating exhaust gas from an engine is disclosed. The muffler includes an outer casing defining a closed space therein and having an inlet pipe and an outlet pipe for introducing and discharging the exhaust gas into and out of the outer casing, respectively. At least one partition wall is provided so as to divide the closed space of the outer casing into two or more spaces. At least one penetration pipe is provided through each of the partition walls for placing the divided spaces in communication with each other, and a catalyzer is provided inside at least one of the penetrating pipes. Exhaust gas noise is thus reduced and at the same time toxic substances such as NO.sub.x or SO.sub.x are removed by the muffler by a single structure.