the space below the track rail 120 the braces 10 are connected again and 310 are deformed as following namely the ring 332 is taken out from the hook 331 next the lower brace 312 is rotated in the anti clockwise direction in fig6 b with the rotation shaft 312 a as the rotation center next when the lower brace 312 is rotated until the upper end of the lower brace 12 approaches the lower end of the upper brace 11 the upper end of the lower brace 12 is connected to the upper brace 11 then the lower cover 22 is fixed to the upper cover 21 in this way by rotating the lower brace 312 with respect to the upper brace 311 it is possible to recover the rack 30 in the space below the track rail 120 as described above according to the second embodiment by rotating the lower brace 312 with respect to the upper brace 311 it is possible to easily perform moving back the rack 30 from the space under the track rail 120 and also easily perform re installing the rack 30 to the space below the track rail 120 further upon recovering the rack 30.