An exhaust emission control system for an engine that provides both early activation of a catalyst during cold-engine, low-load operation of the engine and protection of the catalyst during hot-engine, high-load operation of the engine. The exhaust emission control system includes a catalyst provided in an intermediate portion of an exhaust pipe; an exhaust valve provided in an upstream-side exhaust passage of the exhaust pipe between the catalyst and the engine; and an auxiliary exhaust passage, which is smaller in passage cross-sectional area than the upstream-side exhaust passage and is connected to the upstream-side exhaust passage so as to bypass the exhaust valve. The auxiliary exhaust passage has an inlet connected in the vicinity of an exhaust port of the engine and an outlet connected to the vicinity of the catalyst.