An exhaust gas purifying system (100) includes: a NOx purifying catalyst (34) that is disposed in an exhaust gas flow path (3) to purify nitrogen oxide; and an HC generator (33, 133) that is disposed upstream of the NOx purifying catalyst (34) in the exhaust gas flow path (3) to generate at least one of acetylene, a hydrocarbon with a carbon number of 2 to 5 other than acetylene, and an aromatic hydrocarbon from a hydrocarbon contained in an exhaust gas. In addition, an exhaust gas purifying method using the exhaust gas purifying system includes the step of adjusting an oxygen concentration in the exhaust gas supplied to the HC generation catalyst (33A, 133A) to 0.8 to 1.5 vol % when an air-fuel ratio is stoichiometric or rich.