A filter differential pressure, which is the difference between the inlet pressure and the outlet pressure of a DPM filter (11), is determined, and switching is performed in accordance with the regeneration condition of the DPM filter (11) between a first trapped DPM amount calculation process for estimating the trapped DPM amount after complete regeneration, in which all of the DPM trapped in the DPM filter (11) is burned, and a second trapped DPM amount calculation process for estimating the trapped DPM amount when a part of the DPM trapped in the DPM filter (11) has been burned away. The trapped DPM amount in the DPM filter (11) is estimated on the basis of the filter differential pressure using one of the first and second trapped DPM amount calculation processes.