The recent trend in ship design brought out fuller and beamier ships with poorer course stability. Naval architects are anxious about a remarkable loss of propulsive power which will be caused by such poor stability. On the instability criterion of manual steering, we already discussed from the view point of phase compensation ability of helmsman^{1), 2)}. In the present study, the same problem will be discussed on autopilot steering in open sea, utilizing the performance criterion of autopilot navigation^{1), 2)}. In this first report, we provide the method of calculation with some examples and discuss about the results. The numerical calculations were carried out by the frequency response analysis. This method is useful if the system is linear and each component of the system is known. The disturbance consists of wave and wind. Each spectrum and response amplitude operator are reasonably evaluated. By the calculations using E 10 (E Series ship with L / V =10 sec), it is found the following facts. If the system stability becomes inferior by the ship herself or by autopilot coefficients, low-frequency yawing grows by wind. On the other hand, high-frequency disturbance of wave causes large amount of rudder movement according to the increase of rate control in autopilot. However, this steering only acts for resistance and scarcely reduces yawing. Further investigation concerning to the effect of L / V , the relation between the phase margin of the system and the energy loss etc. will be dealt with in the second report.