Abstract: An upper body, forming a large deck which is always above water, and a submerged body, which is always submerged in the water, are disposed in the up-and-down direction with a certain distance therebetween. The upper body and submerged body are connected by a plurality of connecting legs. An outer peripheral portion of the upper body is formed to have a stream-lined configuration symmetric in the up-and-down direction. A middle portion of the upper body is formed to have upper and lower surfaces parallel with one another, except for the outer peripheral portion thereof. The connecting leg may include either a single hollow body or a double hollow body. The submerged body is formed to be approximately a T-shaped in vertical cross-section and has a permanent ballast fixed at bottom and side surfaces of the inside portion thereof. A permanent ballast may also be filled in a lateral connecting body which connects adjacent submerged bodies.