Pedestrians approaching to the entrance to the stairs, are considered to judge the distance to the first descending step, and then adjust a pitch of walking. Relations of dimensions of tread and rise, and human peripheral vision, were theoretically analyzed. A theoretical method for obtaining the required dimensions of a tread was suggested in this paper. Pedestrians, in descending stairs, are usually looking down the stairs to insure a safer footing on a tread. If the pedestrians vision to their foot is obstructed in descending, they are considered to be liable to stumble. Two male subjects participated in the experiments. The human peripheral vision in descending stairs was analyzed in 24 different combinations of tread and rise. The ratio of vision screened by thigh movements was suggested for evaluating dimensions of tread and rise in regard to human peripheral vision.