To prevent deflection, by providing treads, cut strings supporting both ends of the treads, and supports arranged in the longitudinal middle parts to support the cut strings by the wall face of a building frame body.

A stair 10 as an upward passage from a balcony at the second floor of a building to the uppermost floor or a downward passage thereof, is provided with treads 20, cut strings 30, and supports 40. The lower ends of the cut strings 30 is made as a free end and a lower end space 52 is formed between the lower end side floor face 81 positioned at the lower end of the stair 10 and the lower end of the cut strings 30. And the upper end of the cut strings 30 is made free and an upper end space 51 is formed between the upper end side floor face 82 positioned at the upper end of the stair 10 and the upper end of the cut strings 30. Then, supports 40 are provided to be constituted of receive tools fixed to three columns 93 of the wall face 90 positioned at the side of the stair 10 and a support beam extended along the inclined underfaces of the cut strings to support the cut strings 30. Accordingly, the structure of stairs prevents deformation or deflection resulting from the own weight.