EXHAUST GAS PURIFICATION SYSTEM AND EXHAUST GAS PURIFICATION METHOD

Provided is an exhaust gas purification system 1 including a catalyst device 31a configured to recover a purification capacity of a catalyst by an air-fuel ratio richness control, wherein a richness control permission range ± where the air-fuel ratio richness control is permitted to be performed, and a richness control prohibition range ² where the air-fuel ratio richness control is prohibited from being performed are provided; by using a catalyst temperature T of the catalyst device 31a and also an air intake amount A of an internal combustion engine 10 as parameters for determination as to the richness control permission range ± or the richness control prohibition range ², the richness control prohibition range ² is set as a range where the catalyst temperature T of the catalyst device 31a is lower than a preset lower limit catalyst temperature Tc and the air intake amount A is larger than a preset upper limit air intake amount Ac; and further the lower limit catalyst temperature Tc is set to a temperature lower than a catalyst activation temperature Ta. This makes it possible to reduce the HC and NOx slip amounts and raise the catalyst temperature T when the temperature is low.