PURPOSE: To facilitate the change of wiring, by arranging removable connectors on the way of lines.

CONSTITUTION: Between a foundation floor 1 and a surface floor 3, a space corresponding to the height of a bearing unit 2 is formed. At the proper position of the foundation floor 1, a joint box 11 is fixed, and a lead-in line 12 is branched. The tip sections of branched lines 13 are provided with female connectors 5, and to male connectors 6 connected removably to the connectors 5, connecting lines 14 are connected. The other end sections of the connecting lines 14 are provided with plug sockets 7 which are fixed on floor plates 3a and the plug edge inserting port of which can be exposed on the upper surface side. When the arrangement of the plug sockets 7 is changed, then in a state that the female connectors 5 and the male connectors 6 are separated from each other, the plug sockets 7 are arranged at desired positions, and after that, the male connectors 6 on the plug socket sides may be connected to the near female connectors 5. Accordingly, a distance for leading the lines is shortened, and can be easily accommodated to the change of the arrangement of connecting instruments.