PURPOSE: To obtain the swing motion reducing effect while stopping and prevent the increase of the hull resistance while cruising by an arrangement wherein a container capable of being expanded or contracted through the charging or exhaustion of gas is provided between a pair of supporting hulls and thus provided container is expanded while stopping and contracted while cruising.

CONSTITUTION: A swing motion reducing means 6 is provided between a pair of supporting hulls 3 of a twin-hull ship and below and upper hull 2. The swing motion reducing means 6 has an air tight structure like a container. A top wall 6a and a bottom wall 6b of the means 6 are formed by rigid structual materials such as steel plates and peripheral walls 6c are formed by expansion materials such as flexible rubber plates. While normal cruising the swing motion reducing means 6 is contracted and housed. On the other hand, while stopping or cruising at very low speed compressed gas is charged into the swing motion reducing means 6 through a gas charging and exhaustion pipe 8 to expand the means 6 and then form a float, thereby reducing the motion of the twin-hull ship in the vertical direction.