To reduce the size of an oscillation reducer and the loss of a sailing speed for applying the reducer to wide variety of ships including medium and small ships and high-speed ships and to enable application to coupling motion with rolling, bow oscillation and pitching combined for effectively reducing the oscillation in various directions of waves.

This oscillation reducer comprises a vertical fin for generating lift in water in the moving direction and the horizontal direction of a ship, two fins for generating lift in the vertical direction, and a fin supporting mechanism and an incidence angle changing mechanism. It has a mounting portion in use for mounting the whole device at the bottom of the bow. Its has a central barrel outlined in a streamline shape in vertical cross section. The whole reducer is big enough to accommodate itself in a circumscribed rectangular solid of the hull.