To provide a new fastening part structure capable of substantially maintaining a sure close contact state between an end surface of a flange and an end surface of a box nut, in the fastening part structure fastened via the flange fixed to a member separate from a bolt member or a box nut member.

The filtering device has a filter member 16 and a casing for storing the filter member 16. At the idle end side of the filter member 16, a tip 30 being a male screw of a cylindrical member 20 fitted around by a filter 22 is exposed from the flange 32a being an end of the filter member 16, and a cap female screw member 38 is fastened to the tip 30 by inserting the flange 32a and a spacer member 34. A part between the cap female screw member 38 and the spacer member 34 is sealed by an O ring 44, and the spacer member 34 and the flange 32a are fixed by being pressed by a coil spring being an elastic pressing member 36.

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