An object is to suppress the rocking motion of a vessel body with a high level of accuracy by obtaining a required lifting force. A torque calculation section 27 calculates a torque of a fin 11 based on the pressure of a hydraulic cylinder 12. This torque is inputted to an angle of attack computing unit 24 via a torque adjusting unit 28, and based on this torque, a fin angle with respect to a flow of sea water, that is, an angle of attack of the fin is estimated. Then a command generation section 23 generates a fin angle command value .theta.3 for matching the estimated angle of attack .theta.2' with a target fin angle .theta.1. Based on this fin angle command value .theta.3, a flow rate adjusting mechanism 14 controls a flow rate Q of hydraulic fluid applied to the hydraulic cylinder 12, to thereby adjust the angle of the fin 11. Thus, the fin angle is controlled based on the angle of attack, which directly relates to lifting force.