An exhaust emission control system of an engine is provided, which includes a NOx catalyst disposed in an exhaust passage for storing NOx within exhaust gas when an air-fuel ratio of the exhaust gas is lean, and reducing the stored NOx when the air-fuel ratio is approximately stoichiometric or rich. A processor executes a NOx reduction controlling module for performing, when the NOx stored amount exceeds a given amount, a NOx reduction control in which a fuel injector performs a post injection of fuel to continuously control the air-fuel ratio to a target ratio so that the stored NOx is reduced to be below a given amount, the target air-fuel ratio being a ratio at which the stored NOx is reducible, the post injection causing the injected fuel to combust inside a cylinder, the execution of the control permitted when an engine load is within a medium load range.