In an exhaust path, a HC trapping material which traps HC contained in exhaust gas temporarily, a H.sub.2O trap which traps H.sub.2O contained in exhaust gas and a CO oxidation catalyst are arranged in this order from the upstream side, wherein the H.sub.2O trap is disposed just upstream of and close to the CO oxidation catalyst. H.sub.2O and HC which are components disturbing the activity of the CO oxidation catalyst can be removed efficiently and the adsorption heat and condensation heat of H.sub.2O can be efficiently utilized to raise the temperature of the CO oxidation catalyst so that early activation of the CO oxidation catalyst is accomplished just after an engine starts.