To improve the boarding comfort and maneuvering performance by restricting the rocking of a hull during the sail.

In an asymmetrical catamaran ship having a pair of lower hulls 2 passing through a water surface, a first and a second turnable rock reducing fins 4a, 4b and a first and a second fin driving devices 8a, 8b for turning each fin 4a, 4b are provided on the outside of a bow of a first lower vessel body 2 displaced to the front side and the outside of a stern of a second lower vessel body 2b displaced to the rear side. On the basis of the detecting signal of a gyrocompass 6 as a rock detecting unit for detecting rock of a hull 5, turning angle (angle of attack) of each fin 4a, 4b is computed so as to restrict the rock of the hull 5, and each driving device 8a, 8b is separately driven in response to the angle of attack so as to turn each fin 4a, 4b. Comfortableness of boarding and maneuvering performance of the asymmetrical catamaran ship with the rock reducing fins are thereby improved.