PURPOSE: To provide a stable large-sized polygonal pressure-resistant container of high quality reduced in material cost, the number of working processes and the number of connection places, easy to produce, achieved in the reduction of cost, capable of reducing the probability of the generation of a leak to a large extent and reduced in problems such as the lowering of dimensional accuracy, the generation of strain caused by heat, the breakage of a bent part or the like.

CONSTITUTION: A peripheral side panel 27 formed into a required polygonal shape as a whole by forming wide notch grooves 30a-30f to the outer surface of one thick- walled metal strip plate material at a plurality of places so as to leave an interval in the longitudinal direction of the strip plate material and bending the bottom thin- walled parts 31a-31f of the respective notch grooves into a circular arc shape is connected to one thick-walled metal bottom panel 25, and the connection part 32 of the polygonal peripheral side panel 27 and the bottom panel 25 and both end connection parts 33 of the peripheral side panel are airtightly soldered over the total length. Square rod-shaped metal reinforcing materials 35 are provided to the notch grooves 30b, 30c, 30e, 30f opened by the bending of the polygonal peripheral side panel 27 by soldering.