As the speed of ships becomes higher and the size becomes larger, the problem of traffic control in inland sea or near the port becomes more acute. An automatic device to set the course of ships and the auto- pilot equipped on a ship herself will prevent many traffic accidents in confined waters. But the auto-pilot in present use is so designed that it controls ships in oceans mainly, it is not suitable for ship's control in inland sea or in confined water course. In order that the ship should follow the course in narrower region the control device should be proportional to the deviation of ship's position besides being proportional to the deviation of heading angle from the directed course. The authors of this paper, therefore, assumed the following type of ship's steering in which δ= k ₁ (θ _i -θ₀) + k ₂ (η _i -η₀) θ _i , η _i : Values denoting the angle and the position of the directed course respectively θ₀, η₀ : Values denoting the actual angle and position of the ship respectively δ : Rudder angle In other words the rudder angle is proportional to the error of both angle and position of ship. k ₁ and k ₂ are suitably selected to make the system stable, so that the ship follows the directed course. This paper describes the conditions to be fulfilled for the automatic steering system assumed by the authors.