In exhaust gas purifying apparatus and method for an internal combustion engine, a degree of a performance deterioration of an NOx trap catalyst is determined on the basis of a detected air-fuel ratio variation time difference, at the performance deterioration determination of the NOx trap catalyst, the degree of the performance deterioration with at least one of correcting factors of a quantity of NOx trapped by means of the NOx trap catalyst, a reducing agent supply quantity by which a reducing agent is supplied to the NOx trap catalyst, and catalyst temperature which is a temperature of the NOx trap catalyst, and at least one of the correcting factors is reflected on the determination of the degree of the performance deterioration of the NOx trap catalyst.