An underwater-side structure that is installed at a sea bed and that supports an above-water structure that is provided above a water, the underwater-side structure comprising a frame and connection portions provided at the lower portion of the frame: the frame comprising: a plurality of pipe legs made of steel that extend from the sea bed to above the water, each of the plurality of pipe legs being formed with a first space therein, the first space having a portion in which a filling material is filled; and horizontal and diagonal braces (35, 36) made of steel that couple the plurality of the pipe legs, the braces (35, 36) being formed with a second space therein; the frame having three stages of horizontal braces: characterized in that: the portion of the first space is an upper end portion of the plurality of pipe legs close to the water surface or a lower end portion of the plurality of pipe legs close to the sea bed; wherein a natural period of the underwater-side structure differs from a natural period of the above-water structure by added weight and rigidity imparted by the filling material filled in the portion of the first space, and wherein the second space has a portion in which the filling material is filled, and wherein the filling material is not filled in the frame positioned above the second horizontal braces from the bottom including said braces, and the connection portions, or the filling material is not filled in the frame positioned below the horizontal braces of the first stage from the top and the connection portions including said braces. the frame comprising: a plurality of pipe legs made of steel that extend from the sea bed to above the water, each of the plurality of pipe legs being formed with a first space therein, the first space having a portion in which a filling material is filled; and a plurality of pipe legs made of steel that extend from the sea bed to above the water, each of the plurality of pipe legs being formed with a first space therein, the first space having a portion in which a filling material is filled; and horizontal and diagonal braces (35, 36) made of steel that couple the plurality of the pipe legs, the braces (35, 36) being formed with a second space therein; the frame having three stages of horizontal braces: characterized in that: the portion of the first space is an upper end portion of the plurality of pipe legs close to the water surface or a lower end portion of the plurality of pipe legs close to the sea bed; wherein a natural period of the underwater-side structure differs from a natural period of the above-water structure by added weight and rigidity imparted by the filling material filled in the portion of the first space, and wherein the second space has a portion in which the filling material is filled, and wherein the filling material is not filled in the frame positioned above the second horizontal braces from the bottom including said braces, and the connection portions, or the filling material is not filled in the frame positioned below the horizontal braces of the first stage from the top and the connection portions including said braces. characterized in that: the portion of the first space is an upper end portion of the plurality of pipe legs close to the water surface or a lower end portion of the plurality of pipe legs close to the sea bed; wherein a natural period of the underwater-side structure differs from a natural period of the above-water structure by added weight and rigidity imparted by the filling material filled in the portion of the first space, and wherein the second space has a portion in which the filling material is filled, and wherein the filling material is not filled in the frame positioned above the second horizontal braces from the bottom including said braces, and the connection portions, or the filling material is not filled in the frame positioned below the horizontal braces of the first stage from the top and the connection portions including said braces. the portion of the first space is an upper end portion of the plurality of pipe legs close to the water surface or a lower end portion of the plurality of pipe legs close to the sea bed; wherein a natural period of the underwater-side structure differs from a natural period of the above-water structure by added weight and rigidity imparted by the filling material filled in the portion of the first space, and wherein the second space has a portion in which the filling material is filled, and wherein the filling material is not filled in the frame positioned above the second horizontal braces from the bottom including said braces, and the connection portions, or the filling material is not filled in the frame positioned below the horizontal braces of the first stage from the top and the connection portions including said braces.