Conventionally, there is a technique in which a diesel particulate filter (or NOx catalyst) is provided in an exhaust gas discharging path of a diesel engine mounted in a running machine, and exhaust gas discharged from the diesel engine is purified by the diesel particulate filter (or NOx catalyst) (see patent documents 1, 2 and 3). There is also known a technique in which a filter case (inner case) is provided in a casing (outer case) and a particulate filter is disposed in the filter case (see patent document 4).