An on-water structure including a liquefied gas tank and a surrounding structural body, in which the liquefied gas tank is installed, the liquefied gas tank comprising: a tank main body in which a liquefied gas is storable, the tank main body including a plurality of face portions (13, 14, 15) and corner portions between the face portions, the corner portions having less rigidity than that of the face portions; a bottom supporting body (23;28) that supports the tank main body from below the tank main body; and a plurality of side supporting bodies (20;25) that support the tank main body from side of the tank main body, the side supporting bodies being provided on one of the tank main body and the surrounding structural body, wherein the tank main body is configured such that: when the tank main body stores no cargo, a gap (d, D) is formed between each of the side supporting bodies and the other one of the tank main body and surrounding structural body, and the tank main body stands by itself by being supported by the bottom supporting body; and when the liquefied gas is stored in the tank main body, a load of the liquefied gas causes deformation of the corner portions, such that the tank main body expands at least laterally, the side supporting bodies come into contact with the other one of the tank main body and surrounding structural body, and the tank main body is supported by the bottom supporting body and the side supporting bodies. a tank main body in which a liquefied gas is storable, the tank main body including a plurality of face portions (13, 14, 15) and corner portions between the face portions, the corner portions having less rigidity than that of the face portions; a bottom supporting body (23;28) that supports the tank main body from below the tank main body; and a plurality of side supporting bodies (20;25) that support the tank main body from side of the tank main body, the side supporting bodies being provided on one of the tank main body and the surrounding structural body, wherein the tank main body is configured such that: when the tank main body stores no cargo, a gap (d, D) is formed between each of the side supporting bodies and the other one of the tank main body and surrounding structural body, and the tank main body stands by itself by being supported by the bottom supporting body; and when the liquefied gas is stored in the tank main body, a load of the liquefied gas causes deformation of the corner portions, such that the tank main body expands at least laterally, the side supporting bodies come into contact with the other one of the tank main body and surrounding structural body, and the tank main body is supported by the bottom supporting body and the side supporting bodies. when the tank main body stores no cargo, a gap (d, D) is formed between each of the side supporting bodies and the other one of the tank main body and surrounding structural body, and the tank main body stands by itself by being supported by the bottom supporting body; and when the liquefied gas is stored in the tank main body, a load of the liquefied gas causes deformation of the corner portions, such that the tank main body expands at least laterally, the side supporting bodies come into contact with the other one of the tank main body and surrounding structural body, and the tank main body is supported by the bottom supporting body and the side supporting bodies.