PURPOSE: To provide joists in a freely detachable manner.

CONSTITUTION: Continued horizontal bearing edges are provided to both side edges of the lower end, and a plurality of longer sleepers 1 with flat surfaces are placed on the floor in parallel with each other in a state to lift them by using height controlling legs erected to sides at specific intervals. Narrow joists 2 provided with spaces into which the height controlling legs are stored at places of them inside the joists and floor boards at other places are built between bearing edges of adjacent sleepers so that the whole surface forms the same plane.