To retain a main top board and a folding top board on the same plane while foldably coupling the main top board and the folding top board.

The top board has the folding top board 2 freely foldably coupled to the main top board 1 via a hinge 3. The hinge 3 couples a second fixed plate 12 to be fixed to the folding top board 2 to a first fixed plate 11 to be fixed to the main top board 1 via a folding shaft 13, couples a first bending plate 21 to the first fixed plate 11 via a first bending shaft 31, couples a second bending plate 22 to the second fixed plate 12 via a second bending shaft 32, and couples the first bending plate 21 to the second bending plate 22 via an intermediate coupling shaft 23. In the top board, the folding top board 2 is fixed within the same horizontal plane as the main top board 1 by retaining a bending plate 20 by a stopper 4 in a posture perpendicular to the fixed plate 10, and is bent in such a posture that the bending plate 20 and the fixed plate 10 are on the same plane to fold the folding top board 2 to the main top board 1.