Concerning regeneration of the diesel engine particulate filter (DPF), study was made on both the external ignition method and catalytic combustion method. As to the former, a diesel fuel burner system and electrical igniter system were evaluated. It was found as a result that the external igniters, unless controlled accurately, give rise to serious problems such as thermal stress damages of DPF and inadequate soot combustion.The catalytic combustion devices evaluated were a catalyzed DPF system and fuel additive system. With these systems it was found impossible to lower the soot ignition temperature (the temperature at which regeneration begins) to a range of exhaust temperatures produced during normal vehicle operation.To overcome these difficulties study was made on another catalytic approach in which solution of a catalytic component is sprayed and supplied directly on the soot-trapping DPF. By setting optimum conditions including the kind of catalyst component, the amount of catalyst, and the type of the DPF, it was possible to bring soot ignition temperature down to exhaust temperatures attainable by normal driving and to realize excellent soot combustion. This system is simple in construction compared with other systems and has another merit of lessening, without using complex control, the possibility of the DPF failing due to thermal stress.