a body on which a close contact groove is concavely formed to form a semicircular plane on a rear surface of a body part with a rectangular parallelepiped shape, wherein a (“shape and a”)” shape are respectively formed to protrude from a rear surface of the body part corresponding to opposite sides of the concave groove and a pair of threaded coupling parts are provided on outer circumferential surfaces of the body part;

a cap having an open front surface and fastened to a pair of coupling parts provided in the body via a thread formed on an inner circumferential surface thereof; and

a shaft provided with a contact plate whose surface is concaved in an arc shape; and a fixation shaft mounted on a rear surface of the contact plate to be rotatable inside the cap,

wherein a through hole is formed by boring a center of the cap, a rotation groove is formed along an edge of the through hole on a rear surface of the cap, and a shaft support member performing a bearing function is configured to be coupled to a fixation shaft of the shaft in a state of penetrating the through hole, and

wherein the shaft support member is integrally formed with a disk-shaped holding plate on a rear surface of the cylindrical body to penetrate the through hole in a state of being interrupted with a rotation groove of the cap, and a pair of coupling holes are formed to correspond to each other and perforate the cylindrical body so that a pair of coupling protrusions provided on the fixation shaft of the shaft are inserted into and interrupted by the coupling holes.