A plurality of catalytic units (42) and (43) are disposed in series with each other in a direction conforming to the direction (X) of flow of the exhaust gases (G) and include respective honeycomb carrier structure each having a catalyst deposited thereon. Of the plural catalytic units, at least two catalytic units (42) and (43), which are positioned downstream and upstream relative to each other with respect to the direction (X) of flow of the exhaust gases (G), respectively, are such that the downstream catalytic unit (42) has a higher cell density, a smaller capacity of the honeycomb carrier structure and a larger amount of an element, due to become a major catalyst, carried thereby per unitary capacity than those in the upstream catalytic unit (43).