Development of rudder gap flow blocking device for lift augmentation and cavitation suppression is presented. In order to verify the performance of this device, cavitation visualization and surface pressure measurements were carried out in a cavitation tunnel. Numerical simulations were conducted using a computational fluid dynamics code for more rigorous verification. The new rudder system is equipped with cam devices, which effectively close the gap between the horn/pintle and movable wing parts. The experimental and computational results show that the proposed rudder system is superior to the conventional rudder systems in terms of the lift augmentation and cavitation suppression.