To efficiently assemble or replace many kinds of form devices for staircase differed in height of a riser face 14 as requested while accurately positioning a number of formboards 11.

Each L-shaped sectional formboard 11 includes a tread face 12 matched to a design depth dimension and the riser face 14 of a dimension more than a design height, which are continued to each other through a polygonal line 15, the angle between the both being variable with the polygonal line 15 as an axis. Side ribs 16 provided near both ends of all the formboards 11 are fixed to a pair of positioning plate 18 to determine an inclination and a space between the formboards. Back ribs 24 of all the formboards 11 are fixed to the riser faces 14 of the adjacent formboards 11. The positioning plates 18 are erected in parallel to each other, both side surfaces of the formboards 11 are covered with side plates 30, and concrete is poured thereto.