PURPOSE: To easily and uniformly set the positions of risers and runs over the width direction with high accuracy regardless of the size of width dimension by avoiding the formation of the risers and the runs by means of a placing form made of a metal for cost reduction, and by facilitating finishing such as tiling in constructing a cast-in-place concrete staircase.

CONSTITUTION: Previously produced at a factory is a plurality of combinations 16 constituted by welding a step-receiving metallic material 8 having a plurality of fixed legs 6 and a ruler receiver 7 to rulers 10 having run corresponding parts and riser corresponding parts. The combinations 16 are mounted on a plurality of declined beams 5 for fixing. A rear step form 9 is installed between the declined beams 5 to close an opening. A formwork riser board 12 is fixed to the front of the riser corresponding of each ruler 10. A staircase is constructed by placing concrete 19 in the upper part of the rear step form 9 with the opening between the riser corresponding parts closed in the rulers 10 adjoining to each other with the formwork riser board 12.