EXHAUST GAS PURIFICATION SYSTEM FOR INTERNAL COMBUSTION ENGINE

An object is to provide an exhaust purifying system for an internal combustion engine having an NSR catalyst, which can inhibit emissions from being aggravated by blow-by of NOx. An exhaust purifying system for an internal combustion engine 10 capable of a lean burn operation is provided. The exhaust purifying system includes: an NSR catalyst 16 disposed in an exhaust passage 12 of the internal combustion engine 10; an SCR 18 disposed downstream of the NSR catalyst 16; a NOx sensor 26 disposed downstream of the SCR 18, the NOx sensor 26 producing an output according to an NH 3 concentration; and rich spike means for performing a rich spike. The rich spike means starts the rich spike at a predetermined timing during a lean burn operation and terminates the rich spike at a timing when the NOx sensor 26 issues a predetermined output characteristic indicative of a rise in the NH 3 concentration.