In an exhaust emission control system for an engine, comprising an exhaust emission control catalyst disposed in an exhaust muffler whose inside is divided into a plurality of chambers, at least a portion of the inside of the exhaust muffler is divided into a first chamber for converting mainly NOx in exhaust gas and a second chamber for converting HC and CO in the exhaust gas. The exhaust muffler comprises: a partition wall having a conduit for guiding the exhaust gas from the first chamber to the second chamber; an exhaust gas inlet for introducing the exhaust gas into the first chamber; an external-air suction means for sucking external air into the second chamber; and a discharge pipe for discharging the external air and the exhaust gas flowing through the second chamber.