An exhaust gas purification device for an internal combustion engine

An exhaust gas purification device for an internal combustion engine which operates at a lean air-fuel ratio and at the stoichiometric air-fuel ratio or at a rich air-fuel ratio as required, comprising: an exhaust gas purifying catalyst (5) disposed in an exhaust gas passage of the engine and having an O2 storage capability; an NOx occluding and reducing catalyst (7) disposed in said exhaust gas passage on the downstream side of said exhaust gas purifying catalyst (5) to absorb NOx when the air-fuel ratio is lean and to release the absorbed NOx when the oxygen concentration in the exhaust gas flowing in has decreased; an NOx occlusion amount-estimating means for estimating the amount of NOx occluded by said NOx occluding and reducing catalyst (7); and an NOx occlusion amount correction means for correcting the estimated occluded amount of NOx based on the amount of oxygen stored in catalyst (5).