A filter (14) traps particulate matter contained in the exhaust gas of a diesel engine (1). The filter (14) is regenerated by burning the trapped particulate matter through an operation to raise the exhaust gas temperature. An oxidation catalyst which promotes combustion of the particulate matter is coated onto the filter (14). The oxygen concentration of the exhaust gas upstream of the filter (14) and the oxygen concentration of the exhaust gas downstream of the filter (14) during the regeneration period are detected using universal exhaust gas oxygen sensors (17, 16). A controller (25) determines a substantial regeneration period on the basis of the difference between these oxygen concentrations, and by comparing a maximum value of the difference between the oxygen concentrations during the substantial regeneration period with a predetermined threshold, determines deterioration of the filter (14) with a high degree of precision.