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.