The classical PID control law is still used for many marine control systems. Gain tuning in the controller is, however, a formidable task for mariners. This paper provides a simple and a safe automatic P, I, D gains tuning method using relay control, for typical marine controllers with PID control law. In this method, a limit cycle with small amplitude is occurred, at the beginning, through a proper actuator such as a rudder, a thruster and so on, and then the amplitude and the period of the limit cycle are observed. Since the two quantities obtained under such limit cycle equivalently correspond to those by the ultimate method (Ziegler Nichols scheme), these are utilizable as the parameters for obtaining the optimal gains recommended by Ziegler Nichols. The on-board computer control system using the above method is applied to typical three types of ship's controller ; (1) autopilot system, (2) yaw control system through bow thruster and (3) diesel engine governor system, and the results of the full scale experiments using a small training ship are discussed.