T. H. Havelock and H. Maruo dealt with the problem of the mean wave pressure, or so-called steady drifting force, acting on fixed bodies amongst regular waves. There lies, however, great analytical difficulties, and in general the first approximation gives no fair result. Meanwhile, this problem is greatly simplified under the restriction that the body has wallsided infinite draft, for the local disturbance cancels out and the diverging wave only is left, and the problem becomes two dimensional. The author has solved numerically the integral equation, which is the boundary condition on the body surface, firstly in the case of a circular cylinder as a trial of the method to be used here, and secondly in cases of three wall-sided parabolic waterline ships with infinite draft. ( B/L =0.10, 0.15, 0.20) The results have shown that the first approximation is wrongly fitted to the calculated source distribution, and to give the steady drifting force by its approximation is hopeless. The steady drifting force obtained is nearly proportional to the square of the beam in the range λ/ L =16, and has maximum value between λ/ L =23. (see Fig.4)