To provide a method for controlling a NOx reduction system which can remove NO x with a satisfactorily high efficiency even under low-temperature operating conditions as in diesel cars. The NO x reduction system comprises NO x treatment means disposed in an exhaust passage in an internal combustion engine, and air-fuel ratio control means for bringing the air-fuel ratio of an exhaust gas to a lean or rich state. The NO x treatment means comprises at least a first catalyst layer containing a solid acid catalyst capable of adsorbing ammonia and a second catalyst layer containing a noble metal and a cerium oxide material. The second catalyst layer and the first catalyst layer are stacked in that order on a carrier, and the first catalyst layer constitutes the uppermost layer. The air-fuel ratio control means controls so that the lean state and the rich state are repeated at predetermined intervals. According to the above constitution, NO x is passed through the first catalyst layer in the lean state and is temporarily stored in the second catalyst layer. Thereafter, in the rich state, NO x is converted to NH 3 and is restored in the first catalyst layer. When the state is again returned to the lean state, NH 3 is converted to nitrogen which is finally released from the first catalyst layer.