In order to manoeuvre a gigantic ship easily, some devices such as auto-pilots and yaw rate indicators have been equipped on board to keep and/or alter ship's course. Also, for the abovementioned purpose, there are some studies concerned with a time optimal control to alter ship's course [1, 2, 4] and an improvement of ship's manoeuvrability by means of a subsidary automatic device [5, 10]. This paper deals with a subsidary automatic control system which is a kind of the time optimal steering for ship's steady turning. The proposed control system has a yaw rate input instead of a rudder angle which is given by helmsman through a steering wheel, and has a feed back loop of yaw rate and can improve the quick responsibility of a ship to almost maximum degree without destroying turning ability. It is, therefore, expected that the helmsman can control easily and quickly even for a ship whose course stability is unstable. In this paper, the required time to set the ship's turning steady by the automatic control system is compared with the times by the time optimal steering and the ordinary steering by using of the digital computer simulation of ship's manoeuvre.