PURPOSE: To rotate a blocking plate with clamping screws loosened, to open an operating hole and to operate in the hole by rotatably coupling the one end of the plate for blocking the hole to a floor, and detachably coupling the other end to the floor.

CONSTITUTION: A blocking plate 9 is provided on an operating hole 4, its one end is rotatably coupled to the floor via two hinges 10, and the other end is detachably coupled to the floor via two set screws 11. In this case, the screws are first loosened, the plate 9 is opened as indicated by imaginary lines, a control cable under the floor is disposed along the longitudinal direction of the hole 4, disposed on a cable hanger 8, and led into a switchboard from plug-in ports 3a, 3b. Thus, when the drawing of the control cable is finished, the plate 9 is closed, and the screws 11 are clamped. As a result, the drawing operation can be conducted merely by opening the plate without removing.