When particulate matter (PM) has accumulated on a diesel particulate filter (DPF), multiple-injection, post injection, and exhaust pipe injection are performed such that a reduction gas is burned in a diesel oxidation catalyst (DOC) to raise the exhaust gas temperature, burn the PM accumulated on the DPF, and regenerate the DPF. A light-off performance map is prepared, in advance, by determining burning efficiency relative to the DOC temperature during the post injection and the exhaust pipe injection, both when the catalyst is fresh and when the catalyst has deteriorated. The DOC temperature suitable for the exhaust pipe injection is set from a degree of the catalyst deterioration on the basis of the light-off performance map. The multiple-injection and/or the post injection is performed when the DOC temperature is less than the set temperature, and the exhaust pipe injection is performed when the DOC temperature is equal to or greater than the set temperature.