To effectively utilize a space in a building in such a manner that spiral stairs are partially constituted by providing an overhanging part, which receive a protrusive plate part of a spiral treadboard equipped with the protrusive plate part protruding to an outdoor side from an exterior wall of the building, in the state of making the overhanging part overhang to the outdoor side from the exterior wall of the building.

This building A is equipped with the spiral stairs B which are used so that a person can come and go between upper and lower floors Aa and Ab while changing an ascending/descending direction in a spiral treadboard part Ba. The spiral treadboard Ba', which is provided in the spiral treadboard part Ba, is equipped with the protrusive plate part Ba" which is protruded to the outdoor side from the exterior wall section Ca' provided on the foundation of the building A. The overhanging part E, which receives the protrusive plate part Ba" of the spiral treadboard Ba' and in which a motion space Ea for enabling the person to ascend/descend on the spiral treadboard Ba' is provided on the received protrusive plate part Ba", is provided in the state of overhanging to the outdoor side from the exterior wall section Ca'.