In order to solve this problem, the Ba solution, the K solution, the Sr solution, and the Mg solution are divided into two groups including a group which is impregnated into the alumina layer first, and a group which is impregnated later, and the K solution is impregnated later. [0063] In this case, because K neither forms a complex nor undergoes affinity with the other NOx absorber, it is not ess