A pressure vessel having reduced weight while satisfying required design conditions has polygonal upper and bottom walls, a side wall provided therebetween, an upper curved surface portion for connecting the upper and side walls, and a lower curved surface portion for connecting the bottom and side walls. An upper wall center rib includes an upper wall center part, and upper wall annular ribs respectively connecting substantially center parts of the upper wall and surrounding the upper wall center rib. Ribs are formed substantially centrally in the upper curved surface portion. A bottom wall center rib includes a center part in the bottom wall. Bottom wall annular ribs are formed connecting the bottom wall and surrounding the bottom wall center rib. Lower curved surface portion ribs are formed substantially centrally in the lower curved surface portion, and main surface ribs are formed substantially centrally in the side wall.