To form the rail of a given length by connecting a great number of units as required where the rail is formed not out of one standard length rail, but out of each short length unit.

An engaging hole 2 about one half of the side of a thick square plate 1 in diameter, is provided for the center of the thick square plate 1, each T type recessed groove 3 is provided underneath the aforesaid plate at the same time, in which its width is roughly identical to the diameter of the aforesaid engaging hole, and its depth is roughly one half of the thickness of the thick square plate 1, an overhang plate 5 in a U shape where its width is almost to be fit in the groove, its thickness is identical to the depth of the T type recessed groove, and its tip end is in a semi-circular shape, is provided at the center lower part of the one side 4 of the thick square plate 1 to which no T type groove 3 is faced, concurrently a boss 6 which is identical in height to the upper surface of the thick square plate 1, and also identical to the engaging hole 2 in diameter, is extended to a position parted by a distance from the one side 4 to the engaging hole 2, and a through hole 7 is bored at the center of the boss 6.