To enable at least a lid body to be reused among constituent members and at the same time, also enable a case to be reused through replacing at least a filter element among inner members by constituting the case of a case body and the lid body to be mounted on the opening part of the case body.

When a filter for oil treatment reaches its use limits, it is removed from an introduction pipe on the engine side and a lid body 11b for a case 11 is removed from a case body 11a. Then a valve body 7 can be removed from a filter element 2. Consequently, a relief valve 5 with a leaf spring for retaining the filter element 2 consisting of the leaf spring 5a for retaining the filter element 2 and a relief valve body 6 combined in one piece can be stripped and separated all at once from the filter element 2 by unloading the relief valve 5. When the lid body 11b is installed, in a freely attachable/ detachable manner, on the case body 11a as described, the case 11 can be reused only by replacing the filter element 2 and especially the lid body 11b is reused as often as it can be by replacing the valve body 7 or the like.