An auto-pilot affects the ship motions either of an actual ship in the oceans or of the model ship in the experimental tanks. Especially for a high-speed container ship the rolling motions induced by the auto-pilot steering should not be ignored, because the rudder forces are large enough to heel herself owing to her high-speed and relatively small GM value. The rudder movements can be treated as the seventh degree of freedom of ship motions in waves which couples with the other motions. The authors have described the equations of motions about rolling, yawing and swaying motions including the rudder forces which are induced by the periodic movements of the rudder of the auto-pilot. The model experiments in the oblique regular and irregular waves as well as the sine steering experiments in the still water have been carried out and the experimental results have been compared with the theoretical values. It is concluded that; a) Rolling motions by sine steering in still water can be calculated with a sufficient accuracy by the strip theory, b) The effects of auto-pilot should be taken into account to calculate the lateral ship motions as rolling, yawing and swaying motions in regular oblique waves, especially in the quartering waves, and c) The auto-pilot gives a ship the restoring moment of yawing motion which induces the synchronous yawing motions with very low frequencies in irregular waves.