PURPOSE: To reduce the required labor and time by providing stair members at supporting beams previously, and constructing floors and stairs alternately and serially.

CONSTITUTION: A stair 9a and stair members 3, 3a are suspended to the upper side of a stair 9, and floor lateral bars 5a, 5b and wall lateral bars 7a, 7b of supporting bars are fixed at the supporting bars 1, 2 respectively. Next the supporting bars 1, 2 and the floor lateral bars 5a, 5b and supporting bars 1, 2 are moved up for one floor in the direction of an arrow 14 respectively. The supporting bar 1 is then fixed with the floor lateral bars 5a, 5b, the supporting bar 2 is fixed with the wall lateral bars 7a, 7b, and then a stair 9b and the stair member 3a are suspended to the upper side of the stair 9a and installed temporarily. After a floor is constructed, bolts at fixing parts of the supporting bars 1, 2 are removed, and the supporting bar 1 is put up for one floor and fixed. A necessary number of stair members 3, 3a are provided temporarily serially over the constructed stair members 3, 3a, and the floor is constructed.