A spray device 40 sprays a liquid oxidation catalyst 41 into an interior of an upstream small-diameter pipe 32 which is disposed upstream of an oxidation catalyst 25, so that the liquid oxidation catalyst 41 is supplied to particulate matters which have accumulated on an upstream end face 31 of the oxidation catalyst 25 and a diametrically expanded pipe 33 which is disposed upstream of the upstream end face 31 to thereby oxidize and remove the accumulated particulate matters by a catalytic action of the supplied liquid oxidation catalyst 41, whereby it becomes possible to remove the particulate matters which have accumulated on the upper end face 31 of the oxidation catalyst 25 which is disposed in an exhaust passageway and a portion of the exhaust passageway which is situated upstream of the upstream end face 31.