A stairway in homes is an important structure for movement between floors. However, the number of fall accidents on stairways is very high in Japan. The present study aimed at investigating the dimensions of safe stairways especially by subjective assessments and three-dimensional motion in both sexes and three age's groups. Eighteen healthy men and women aged between 21 and 75 years climbed up and down five kinds of stairs at different dimensions (rise and tread of a step). While going up the stairs, the height of a step influenced the track movement of their toes, which decreased distances between the toes and edge of the stairs, especially in the elderly group. Furthermore, the female group trended to feel more subjective strain with a larger height of a step. While going down stairs, a smaller tread of a step decreased the contact area of the soles on the surface of the step. These findings suggest that requirements of stair's dimensions for preventing fall are different for climbing up and down, and for users of different ages and sex.