a body comprising a receiving part of which upper and lower portions have an opening, an upper guide part, and a lower guide part;

a moving member disposed in the receiving part;

a screw which is screw-coupled to the body and moves forwards or backwards relative to the body integrally with the moving member when rotating;

an upper plate which has a guided part guided by the upper guide part and is disposed above the body while being engaged with the moving member; and

a lower plate which has a guided part guided by the lower guide part and is disposed under the body while being engaged with the moving member,

wherein the body further comprises a screw hole formed in a front of the body, and the screw comprises a head part rotatably coupled to the moving member and a screw thread part screwed to the screw hole, and

wherein the moving member comprises a screw coupling hole formed so that the head part penetrates and is coupled to a front side of the moving member.