To simplify mounting/demounting work without dropping a bolt by mounting a bolt retaining means in a groove formed in a penetration part of a bolt, and enabling it to be held in a through hole of a machine body.

An O ring 3 is mounted in a groove formed in a penetration part 5 of a bolt 1. The ring 3 has a slightly larger outer diameter than an inner diameter of a through hole 13 of a machine body 11 while its inner diameter is slightly smaller than an outer diameter of the groove of the penetration part 5. Accordingly, the bolt 1 is compressed in the diameter direction of the ring 3 within the hole 13 when inserted thereinto, and retained in the hole 13 by the aid of frictional force between the ring 3 and the hole 13 generating owing to the repelling force against the compression. Also, when a bolt is unfastened until engagement between a screw part 4 and a screw part 14 of an adaptor 12 is released, similarly with the case of mounting, it is retained within the hole 13 by the aid of the frictional force between the ring 3 and the hole 13.