The hydroelastic response is significant from the viewpoint of the structural design of a Mega-Float. To equip a Mega-Float with some attachments, such as vertical plates, is one of the ways to reduce the hydroelastic response easily. Wave drift force acting on the Mega-Float may, however, be increased in general, when the Mega-Float is equipped with the attachments in order to reduce the response. In the present study, we developed a novel attachment that was effective in the reduction in both hydroelastic response and wave drift force. Further we estimated the response of Mega-Float equipped with the attachments, and compared the result with the data measured in the at-sea experiment. As a result, we confirmed both the effectiveness of the novel attachment and the validity of the estimation method.