The author proposed the entirely new ship's autopilot system using a stochastic autoregressive model (AR autopilot) and corroborated that the system worked well in the actual sea test by a small training ship (about 310 g. ton). Many sea tests of the AR autopilot has been carried out by the same ship since the first trial. Emphasis of the tests was put on (1) the effects that the change of gains gives to the course keeping quality, (2) the robustness to the change of the disturbance, (3) the method to eliminate the offset from the desired course and so on. In this paper, the author analysed in detail the results gained at the above sea tests from the view points of the course keeping quality of the AR autopilot.