A hydrocarbon trapping layer (25) is formed by coating a hydrocarbon trapping material onto a carrier (21A). A number of exhaust gas passages (34) are formed in the carrier (21A) and a plurality of slits (22) are formed on the carrier (21A) to cross the passages (24) in a transverse orientation. The slits (22) delay temperature increases in the downstream hydrocarbon trapping layer (25) and delay the release of trapped hydrocarbons therefrom. This is due to blocking the transmission of heat to the downstream hydrocarbon trapping layer (25) from the upstream hydrocarbon trapping layer (25). The slits (22) further produce a turbulent flow in the flow of exhaust gas and promote diffusion of exhaust gas over the entire surface of the carrier (21A).