When an internal combustion engine is in a low rotation speed driving condition, a duration of the low rotation speed driving condition is measured, and when this duration exceeds a predetermined time, the forced regeneration control is performed. Thus, even when an exhaust flow rate is small and an estimation of a particulate matter accumulated amount due to a pressure difference between front and back sides of a filter is difficult, a forced regeneration time can be accurately judged, the deterioration of fuel efficiency that is caused by excessive trapped particulate matter can be avoided, and thermal runaway that is caused by the excessive particulate matter, and the melting damage of the diesel particulate filter due to this thermal runaway, can be prevented.