A diesel particulate filter (41) traps particulate matter contained in the exhaust gas of a diesel engine (1) for a vehicle. The filter (41) is regenerated by raising the exhaust gas temperature so that the trapped particulate matter burns. A controller (31) calculates a particulate matter combustion amount PMr during regeneration, and in accordance with increases in the combustion amount PMr, raises the oxygen concentration of the exhaust gas by operating an intake throttle (42) and/or a variable nozzle (24) of a turbocharger (21). As a result, the filter (41) is held at an optimum temperature for regeneration regardless of the residual particulate matter amount, and hence the time required for regeneration can be shortened without damaging the heat resistance performance of the filter (41).