and 7B are plan views showing the states of a board table with no spacers mounted thereon and with spacers mounted thereon, respectively; FIGS. 8A and 8B are sectional views taken on the lines8A--8A and 8B--8B in FIGS. 7A and 7B, respectively; FIG. 9 is a sectional view taken on the line 8B--8B in FIG. 7B.In FIGS. 5 and 6, reference numeral 1 denotes surface mounting components of the same size mo