EXHAUST GAS PURIFICATION SYSTEM AND EXHAUST GAS PURIFICATION METHOD

When a catalyst temperature T of a catalyst device 31a is at or below a lower limit catalyst temperature Tc, the air-fuel ratio richness control is prohibited. When a first timing ti1 where an estimated value V of a NOx storage amount has reached an enrichment start threshold value Vc and a second timing ti2 based on a set interval time tic in an enrichment interval time map M1 are both satisfied, the air-fuel ratio richness control is started. The second timing ti2 is corrected by multiplying the set interval time tic by an enrichment interval correction coefficient ± preset based on the catalyst temperature T and on a storage ratio R of the estimated value V of the NOx storage amount to an enrichment start threshold value Vc of the NOx storage amount. With this, the frequency of the air-fuel ratio richness control of a catalyst device 31a configured to recover a purification capacity of a catalyst by an air-fuel ratio richness control is reduced, and the catalyst temperature T is raised while preventing the white smoke development and the HC slip, to thereby achieve improvement in exhaust gas composition and improvement in fuel efficiency.