An upstream cylinder (21) and a downstream cylinder (24) are fixedly mounted to a support member (19), and a filter cylinder (26) is provided mountably and dismountably between the cylinders (21) and (24). In consequence, only the filter cylinder (26) can be dismounted with the cylinders (21) and (24) left as they are on the support member (19), so that the maintenance operation of a particulate matter removing filter (28) can be performed easily. In addition, pressure conduits (32) and (33) and a pressure sensor (34) of a pressure detection unit (31) for detecting a clogged state of the particulate matter removing filter (28) are disposed at positions offset from a moving path (30) of the filter cylinder (26). In consequence, only the filter cylinder (26) can be easily mounted or dismounted without being obstructed by the pressure conduits (32) and (33) and the pressure sensor (34).