In an internal combustion engine, an exhaust passage is provided with a NOx storing and decomposing catalyst (20). When burning fuel under a lean air-fuel ratio, the nitrogen monoxide is adsorbed by the NOx storing and decomposing catalyst (20) and disassociated. At this time, the disassociated oxygen is held in the form of oxygen ions O− on the NOx storing and decomposing catalyst (20). By making the air-fuel ratio rich and purging part of these oxygen ions O−, the remaining oxygen ions O− are also purged. Due to this, the NOx purification performance of the NOx storing and decomposing catalyst (20) is restored.