A pressure vessel having reduced weight while satisfying required design conditions has polygonal upper and bottom walls, a side wall between the upper and bottom walls, an upper curved surface portion connecting the upper wall's peripheral edge and the side wall's upper end wall by curved surfaces convexed outward, and a lower curved surface portion connecting the bottom wall's peripheral edge and the side wall's lower end by curved surfaces convexed outward. The upper wall center portion is thicker than the upper curved surface portion, which is thicker than an upper wall annular portion formed between the upper wall center portion and the upper curved surface portion. The bottom wall center portion is thicker than the lower curved surface portion, which is thicker than a bottom wall annular portion formed between the bottom wall center portion and the lower curved surface portion.