Apparatus and method for exhaust gas purification of an internal combustion engine

In exhaust gas purifying apparatus and method for an internal combustion engine, an abnormality determination of an NOx (Nitrogen Oxides) removing catalyst is executed on the basis of output values of both of a first exhaust gas atmosphere detecting section and a second exhaust gas atmosphere detecting section (for example, oxygen concentration sensors interposed in an exhaust passage of the engine between inlet and outlet portions of the NOx removing catalyst) from a time at which an output value of the first exhaust gas atmosphere detecting section is varied to a first predetermined value to a time at which the output value of the second exhaust gas atmosphere detecting section is reached to a second predetermined value when an exhaust gas atmosphere varying section (control unit) increases the ratio of a reducing agent in the exhaust gas.