Side wall effects of towing tank on the responses of a ship in regular head waves are examined theoretically. Ship motions in regular head waves are assumed to be solved by 0. S. M. and the waves reflected by the tank wall are assumed to act on ship's surface as the secondary exciting force and moment. The velocity potential of the reflected waves initially generated by the ship motions are obtained 3-dimensionally by introducing the thin ship assumption. Some examples of numerically calculated results are shown and the quantitative characteristics of the side wall effects on ship motions are explained clearly.