A projected rim (231) is provided on the side surface of a coupler (20), and the step (53) of a cylindrical member (50) is abutted onto the upper surface of the rim (231) so as to engage the cylindrical member (50) with the coupler (20). The fitting projection (312) formed on a rise part (31) is fitted into a fitting hole (51) in the cylindrical member (50) so that the cylindrical member (50) can be so installed as to be spread over the side surfaces of the coupler (20) and the rise part (31). Thereby, the coupler (20) is hard to be removed.
