To solve such problems that missing of his/her footing and slip down accidents occur due to staircases where a sufficient tread dimension cannot be secured with restricted building areas and layout, and that it is required to provide a safe and secure staircase even under conditions that a staircase space is difficult to expand and a large amount of cost is required.

Step boards of a staircase are classified into two kinds in order to substantially widen the treads of the staircase. Each of first step boards is widened in both end parts of a tread and recess-shaped, while each of second step boards is widened in a central part of a tread and projection-shaped, and a number of these two kinds of step boards are alternately arranged for each stage. Accordingly, one foot is placed on a widened tread while the other foot is smoothly moved on a not widened tread and reaches a widened tread of a next upper or lower stage, thus allowing sequential and safe ascending/descending.