To provide an exhaust gas emission control system capable of reducing the installing space of an exhaust gas emission control device while maintaining the cleaning performance of NOx and an exhaust gas emission control method using the exhaust gas emission control system.

A DOC 111, a DPF 121 and a SCR catalyst 131 are arranged in the exhaust passage of a diesel engine in series and a spraying nozzle 151 supplying ammonium is arranged between the DPF 121 and the SCR catalyst 131. In the DPF 121, a plurality of through-holes are arranged parallel in a longitudinal direction spaced apart by partition walls and one ends of the through-holes are sealed in the honeycomb structure, in the SCR catalyst 131 a plurality of through-holes are arranged parallel in a longitudinal direction spaced apart by partition walls. The ratio of the area of the cross-section perpendicular to the longitudinal direction of the SCR catalyst 131 to the area of the cross-section perpendicular to the longitudinal direction of the DPF 121 is 0.55 or more and 0.90 or less and the area of the cross-section perpendicular to the longitudinal direction of the DPF 121 is 300 cm² or more and 1,000 cm² or less.