To improve steering performance using fluid resistance without decreasing propulsion performance of ship by providing a pair of left and right movable type fins movably to both side bilge portions of a ship protruding from a hull, making the fins capable of being set at angles parallel and perpendicular to water stream.

A support shaft 15 extends from a hull 13 to outside the hull to support a fin 14. A rotating plate 16 fixed to the support shaft 15 in the hull 13 has a piston 17. The piston 17 fits with a rotary cylinder 18. By supplying and discharging hydraulic pressure to and from an oil passage 19 of the rotary cylinder 18, the piston 17 moves within a range of approximately ±90 degrees. Thus, the support shaft 15 is rotated, and the fin 14 rotates approximately ±90 degrees. This construction improves course stability by fixing the fin horizontal to the water flow when sailing outseas. In port regions, steering performance is improved by fixing the fin normal to the water flow.