To reduce the space of housing and transportation by forming a leg attaching recessed part integrally with a top plate and forming a leg hiding recessed groove for putting in a leg by recessedly providing it on the back wall of the top plate.

At leg mounting parts 3 positioned at the four corners of the back surface of the top plate 2, the leg attaching recessed parts for fitting and fixing the base part 5 of the leg 4 are formed integrally with the top plate 2 and the attaching strength of the leg 4 is improved. On the back surface of the top plate 2, the leg hiding recessed groove 7 is recessedly provided from the leg mounting part 3 to an inner part in the direction from the back wall 2b to a front wall and formed and the folded leg 4 is housed inside the leg hiding recessed groove 7 recessedly provided on the back surface of the top plate 2. Since the leg hiding recessed groove 7 is formed in a depth for putting in the leg 4 so as not to be projected from the back surface of the top plate 2, the top plate 2 is turned to a flat surface shape when the leg 4 is folded. Then, since the top plate 2 is in a hollow double wall structure by blow molding and it is not required to increase the thickness of the top plate 2 itself even when the leg hiding recessed groove 7 is formed, miniaturization and weight reduction are performed without damaging the strength of the top plate 2.