The present invention relates to a curve-combined square pressure tank with which curves are combined so as to maintain internal high pressure, improve space efficiency, and reduce the weight thereof, and to a pressure tank, in which planes and curves are combined, formed by connecting flat plate members and curved members, having a plurality of aligned tension members for connecting the flat plate members facing each other, and having stress buffer parts formed at connection parts of the flat plate members and the curved members so as to enable internal pressure to be maintained.