When a ship goes in regular waves, the torque, the thrust and the r. p. s. of the propeller fluctuate about some mean values. These fluctuations, the motion of ship and the characteristics of the driving machine interact each other. The author studied these interactions and the following results were obtained. In the self-propulsion test in waves the motion of ship is fed back into itself through the propeller and the driving machine. For the ship of ordinary type, these feed backs are so little that the motion of ship is not affected by the characteristics of the driving machine. Then there are no differences between the motion of the model in the self-propulsion test and that in the resistance test with the gravity dynamometer. However, the patterns of the fluctuations of the torque and etc. are altered by the characteristics of the driving machine. Therefore, in the self-propulsion test in waves the characteristics of the driving machine are not important matters, when the self-propulsion factors are not affected by the patterns of the fluctuations. When the effects of patterns are not negligible it is necessary to reconsider the self-propulsion test together with the driving machine. The considerations of the actual engine give the fact that the driving machine of the ship is less in firmness and heaviness in comparison with the electric motor used for the self-propulsion model at the Mejiro Basin of the T. T. R. I. (Table 1 & Fig. 4).