so that the lower brace 12 is prevented from being dropped as the structure for connecting the upper brace 11 and the lower brace 12 various structures may be employed as long as the restricting surfaces capable of restricting the movement of the lower brace 12 to be dropped are formed such as the surfaces 13 a and 13 b in fig3 a and 3b the connection cover 20 is provided with an upper cover 21 which is fixed to the vicinity of the lower end of the upper brace 11 and a lower cover 22 which is supported by the upper cover 21 the upper cover 21 and the lower cover 22 are covers to cover the connection portion of the upper brace 11 and the lower brace 12 each of the upper cover 21 and the lower cover 22 is a cylindrical member and is open to both of the up and down directions the upper cover 21 and the lower cover 22 are occlusal with each other within an occlusal area 20 a in the occlusal area 20 a screw threads are formed on each of an outer surface of the upper cover 21 and an inner surface of the lower cover 22 so that the screw threads are engaged or occlusal with each other by rotating the lower cover 22 along a direction d 2 in fig3 b while fixing the upper cover 21 it is possible to fix the upper cover 21 to the lower cover 22 or separate the upper cover 21 from the lower cover 22 the upper cover 21 is fixed to.