Forced regeneration start timing of a diesel particulate filter for purifying particulate matter in exhaust gas is judged based on a comparison of a differential pressure across the diesel particulate filter with a predetermined differential pressure threshold across the filter. The predetermined differential pressure threshold is set by multiplying a reference differential pressure threshold by a coefficient which varies stepwise or continuously depending on a mileage of a vehicle after previous forced regeneration. Consequently, particulate matter can be removed by burning while the amount of particulate matter accumulated unevenly in the filter is still small, and excessive temperature rise in the filter resulting from excessive accumulation of particulate matter during forced regeneration and melting-loss of the filter due to excessive temperature rise can be prevented.