A curve-combined polygonal pressure tank for accommodating a fluid at high pressure therein, the pressure tank comprising: flat members disposed at upper and lower portions of the pressure tank; a first curved member that connects edges of the flat member disposed at the upper portion and the flat member disposed at the lower portion and is formed with predetermined curvature; and a second curved member connecting neighboring curved edges of the first curved member, wherein the curve-combined polygonal pressure tank comprises a stress-buffer portion formed therein to prevent stress discontinuity at a connection portion between the flat member and the first curved member wherein the curve-combined polygonal pressure tank includes a plurality of tension members disposed between the flat member disposed at the upper portion and the flat member disposed at the lower portion; and wherein the outermost tension member among the plurality of tension members is spaced apart from a boundary P at which the first curved member and the flat member contact each other by a constant distance d toward the flat member to form the stress-buffer portion, or wherein the outermost tension member among the plurality of tension members is coupled to a boundary P at which the first curved member and the flat member contact each other and is formed to be thinner than a tension member disposed inward to form the stress-buffer portion, or wherein the outermost tension member among the plurality of tension members is coupled to a boundary P at which the first curved member and the flat member contact each other and is curved with predetermined curvature in an opposite direction to the first curved member or is formed with a bent portion that is bent in an opposite direction to the first curved member, to form a stress-buffer portion. flat members disposed at upper and lower portions of the pressure tank; a first curved member that connects edges of the flat member disposed at the upper portion and the flat member disposed at the lower portion and is formed with predetermined curvature; and a second curved member connecting neighboring curved edges of the first curved member, wherein the curve-combined polygonal pressure tank comprises a stress-buffer portion formed therein to prevent stress discontinuity at a connection portion between the flat member and the first curved member wherein the curve-combined polygonal pressure tank includes a plurality of tension members disposed between the flat member disposed at the upper portion and the flat member disposed at the lower portion; and wherein the outermost tension member among the plurality of tension members is spaced apart from a boundary P at which the first curved member and the flat member contact each other by a constant distance d toward the flat member to form the stress-buffer portion, or wherein the outermost tension member among the plurality of tension members is coupled to a boundary P at which the first curved member and the flat member contact each other and is formed to be thinner than a tension member disposed inward to form the stress-buffer portion, or wherein the outermost tension member among the plurality of tension members is coupled to a boundary P at which the first curved member and the flat member contact each other and is curved with predetermined curvature in an opposite direction to the first curved member or is formed with a bent portion that is bent in an opposite direction to the first curved member, to form a stress-buffer portion.