An exhaust gas purifying system of the present invention comprises a first catalyst which contains a noble metal and reduces hydrocarbons contained in exhaust gas, and a second catalyst which is disposed downstream of the first catalyst and contains .beta.-zeolite. In the exhaust gas purifying system, a mixed gas having an air-fuel ratio of not less than 25 is burned in an internal combustion engine, and the internal combustion engine discharges the exhaust gas having an oxygen concentration of not less than 4% continuously or intermittently.