To facilitate assembling work, to relieve the burden of a worker, to reduce the number of parts so as to reduce a cost and also to suppress the formation of a step in the joint parts of top plates.

Connection recessed parts from which the end parts of connecting rods 18 are exposed are formed on the lower surfaces of the joint parts of the top plates 15, connecting nuts 20 are screwed in the screw parts 18a of the connecting rods 18 of the top plates 15, which are adjacent inside the recessed parts so as to permit the connecting rodes 18 to be freely connection possible and also end part nuts are screwed in the screw parts 18a of outer ends in the connecting rods 18 so as to attain freely fastening possibility. Then, end part legs 1 are connected to intermediate legs 2 being adjacent to them, the intermediate legs 2 positioned in middle parts are connected to the end part legs 1 of opposite ends after that, respective top plate receivers 8 and 9 are fitted on them so as to lace the top plates 15 between them, the ends of the connecting rods 18 are connected by the connecting nuts 20 inside the connection recessed parts on the lower surface of the joint parts of the respective top plates 15 and also the outer ends of the connecting rods are fastened by the end part nuts so as to attain assembling.