A diesel particulate filter (41) which traps particulate matter contained in the exhaust gas of a diesel engine (1) comprises an oxidation catalyst (41A) which exhibits a temperature-raising effect during regeneration of the filter (41). A controller (31) calculates the amount of particulate matter trapped in the filter (41) at the start of regeneration as a first amount, and calculates the amount of particulate matter burned during regeneration of the filter (41) as a second amount (S3, S10, S18). A deterioration factor d of the oxidation catalyst is calculated from the ratio of the second amount and first amount, and a target temperature for the next regeneration of the filter is determined on the basis of this deterioration factor d. Thus deterioration of the oxidation catalyst (41A) is compensated for, and an optimum temperature environment for regenerating the filter (41) is realized.