As the dimension of floating structure increases, hydroelastic response can not be neglected while the response as a rigid body decreases. The bending moment of the deck structure becomes very large due to the hydroelastic response and the wave height is limited to 4 or 5 meters from the viewpoint of structural design. Therefore, the hydroelastic response, especially in longer wave period at which the structural response becomes large, must be reduced for its application to rougher sea area. In the present study, attachment structures, i.e. vertical wall type, L-shape type and slope type, are investigated for this purpose.
　First, the hydraulic model tests were carried out. It turned out that the L-shape type attachment and the slope type attachment are effective in longer period. The numerical analysis shows good agreement with the experimental result.
　Second, the mechanism of the hydroelastic response reduction of the large floating structure by use of the L-shape type attachment was investigated by employing the numerical analysis. The most important element proved to be the force cancellation effect at around the attachment. It is also shown that the drifting force becomes large while the vertical quantities are reduced.
　In the last part of the paper, it is proposed to use the hybrid attachment. It means that the L-shape type attachment effective for the longer period and the vertical-wall type attachment effective for the shorter period are used in combination.