To obtain the wave induced pressure distribution and wave-exciting force and moment upon two dimensional bodies, it is necessary to solve the two-dimensional diffraction problem. Tamura has calculated the wave-exciting forces for two-dimensional bodies of Lewis-form section, using the theory of Grim (based on diffraction potential). Hewever, the full solution of the two-dimensional diffraction problem has not been treated so far. In this paper, the author dealt with the diffraction problem for Lewis-form section, following the theory of Ursell and Tasai. The amplitudes and phase differences of the pressure field, rolling moment, sway and heave forces were calculated making use of this method for two Lewis-forms, excited by a given set of incident waves. Results were compared with the data from tests performed at the University of Tokyo's new Seakeeping Basin. The transmitted and reflected wave height were also calculated for various Lewis-forms, in view of their importance in Ocean Engineering Design.