Pressure vessel

A pressure vessel, having:

• a polygonal upper wall;
• a polygonal bottom wall provided on the lower side of said upper wall;
• a side wall provided between a peripheral edge of said upper wall and a peripheral edge of said bottom wall corresponding to the peripheral edge of said upper wall;
• an upper curved surface portion for connecting the peripheral edge of said upper wall and an upper end of said side wall by curved surfaces convexed outward;
• and
• a lower curved surface portion for connecting the peripheral edge of said bottom wall and a lower end of said side wall by curved surfaces convexed outward, the pressure vessel comprising:
• an upper wall center rib formed so as to include an upper wall center part in center of said upper wall and extend in a specific direction;
• upper wall annular ribs formed so as to respectively connect substantially center parts of adjacent sides of said upper wall and surround said upper wall center rib;
• upper curved surface portion ribs formed in substantially center parts of sides of said upper curved surface portion and provided continuously to said upper wall center rib and said upper wall annular ribs;
• a bottom wall center rib formed so as to include a bottom wall center part in center of said bottom wall and extend in a specific direction;
• bottom wail annular ribs formed so as to respectively connect substantially center parts of adjacent sides of said bottom wall and surround said bottom wall center rib;
• lower curved surface portion ribs formed in substantially center parts of sides of said lower curved surface portion and provided continuously to said bottom wall center rib and said bottom wall annular ribs;
• and
• main surface ribs formed in substantially center parts of main surfaces of said side wall and provided continuously to said upper curved surface portion ribs and said lower curved surface portion ribs.