Recently Ship's autopilot has two different requirements contrary each other depending her sailing sea area ; one is fuel-saving performance and the other is course-keeping performance. In order to meet these requirements many ships equip with adaptive autopilot using modern control theory. Here the problems are lying on the applicability for unstable ships and for the change of ship's velocity, because modern control theory is based on linear control theory. In this paper, we have applied Model Reference Adaptive Control Theory to ship's autopilot, and investigated the influences of nonlinearity of ship maneuverability, the change of ship's velocity and external disturbances. We have introduced the nondimensional adaptive algorithm to cope with the influence of the change of ship's velocity using numerical simulation. Furthermore it is possible to design the stable adaptive algorithm not only for course-changing but also course-keeping mode by applying model reference adaptive control with variable adaptive gain.