To provide an auxiliary top board increasing its effective area, freely changing its set position or direction relative to the main top board, and allowing arbitrary selection of especially the width which is increased on the right or left end side.

The auxiliary top board has a longitudinal first side 2a having a shape fitting the shape of a side 1a on the user's side of the main top board 1 as viewed from above, and a second side 2b axisymmetrical to the first side 2a. The auxiliary top board has widths on both longitudinal end sides, one of which is larger than the other, the first side 2a or the second side 2b is made to abut on the side 1a of the main top board 1 in a state where the upper surfaces are flushed with each other, and the auxiliary top board is detachably mounted to the main top board 1.