It is widely known among seamen that the automatic steering introduces sometimes a sluggish yawing with fairly a long period, say, around 100 sec. This is an inherent oscillation of the whole system which is composed of a ship, steering gear and auto-steering device, being naturally accompanied with rudder movement of the same period. This yawing is not vital from the navigational point of view, but it may cause a considerable loss of propulsive power. An analysis with the equation of motion (Chapter 2) shows that the power loss results largely from an inertial resistance induced by yawing and the resistive component of rudder force. Yawing experiments for free-running models of an oil-tanker and two cargo-boats supported the analysis (Chapter 3 & 4). The incremental resistance is given as Eq. (5) and its rough estimation can be done practically by Eqs. (7) and (8). Applying these results, the power losses under automatic course-keeping for two typical merchant-men, a tanker and a cargo-boat, are estimated as much as 2 to 5% for reasonable adjustment of auto-steering device and in rather an exceptional case it may reach even 20% of the normal power.