PURPOSE: To improve workability by installing semicircular recessed parts in both the side edges of thick and thin plywoods of prescribed thickness, disposing the side edges parallel in a joint type and facing each other at prescribed intervals, and assembling and fixing respective plywoods with spacers inserted into the recessed parts and pipes smaller than a prescribed diameter.

CONSTITUTION: Frame plates 1 made of a 10-15mm thick one plywood body of longitudinal rectangular shape are installed, and semicircular recessed parts 2a are formed on both side edges at regular intervals. The side edges of the frame plates 1 are disposed parallel each other in a joint type, and cones 4 of spacers 3 are installed at circular holes in the recessed parts 2a to face the frame plates 1 each other at the prescribed intervals. After threaded rods 5 are screwed into the cones 4, longitudinal pipes 11 of less than 30mm in diameter and lateral pipes 12 are installed and fixed at the outside of the frame plates 1 by using pipe holding fitting 6. It is thus possible to provide cost reduction, as weight reduction of the whole device, and improve assembly strength because it is unnecessary to install reinforcing frames at the plates.