To provide a swing reducing device which more efficiently suppresses even a great swing.

The swing reducing device includes a propeller 1 rotated by wind power, a tower 2 rotatably supporting the propeller, and a floating body 3 mounting the tower. The floating body structures a floating drilling vessel. The swing of the floating drilling vessel is reduced by rotation of the propeller. Waves are generated by wind in the ocean or the like. When the propeller is rotated by this wind, the swing of the floating body by the waves is suppressed. That is, energy of the wind is converted into damping force through the tower, and the swing of the floating body is suppressed.