Regeneration of a diesel particulate filter

A particulate filter (14) traps particulate matter in the exhaust gas of an internal combustion engine (1). The filter (14) is regenerated by burning the trapped particulate matter in a high temperature atmosphere. A first sensor (17) detects the oxygen concentration of an exhaust gas upstream of the filter (14) as a first concentration. A second sensor (16) detects the oxygen concentration of the exhaust gas downstream of the filter (14) as a second concentration. The controller (25) precisely detects regeneration start and end of the filter (14) by determining that regeneration of the filter (14) has actually started when the difference between the first concentration and second concentration is equal to or greater than a predetermined value (S12), and determining that actual regeneration of the filter (14) has been completed when this difference falls below the predetermined value (S6).