An emission control system for an engine includes a catalyst and an exhaust-gas sensor provided downstream of the catalyst in a flow direction of exhaust gas. The exhaust-gas sensor includes a sensor element that includes a pair of electrodes and a solid electrolyte body located between the electrodes. The emission control system further includes a constant current supply portion that changes an output characteristic of the exhaust-gas sensor by applying a constant current between the electrodes, a rich direction control portion that performs a rich direction control after a fuelling-stop control, and a characteristic control portion that performs a rich responsiveness control during the rich direction control. In the rich direction control, an air-fuel ratio of the exhaust gas is made to be richer. In the rich responsiveness control, the constant current supply portion increases a detection responsiveness of the exhaust-gas sensor with respect to rich gas.