An exhaust gas purifying system for an engine, including a HC trap catalyst disposed in an exhaust passage and operative to trap HC contained in the exhaust gas, and a control unit being programmed to: upon activating the HC trap catalyst after the engine is started, control a target excess air ratio to a preset value smaller than a normal value to which the target excess air ratio is controlled when the HC trap catalyst is activated, to thereby rise an exhaust gas temperature; and increase the target excess air ratio as the temperature of the HC trap catalyst rises, to thereby increase a heat quantity generated upon oxidation of the HC trapped by the HC trap catalyst.