This paper presents a simple method by which we can calculate approximately but enough accurately added mass and damping of twin hull cylinder making a heaving, swaying and rolling oscillation on the surface of a fluid or we can calculate the wave exciting force working upon this twin hull cylinder when it is held fixed on incoming plane waves if we know those of one cylinder of the twin hull cylinder. And it is known from the previous paper by one of the authors that the accuracy of added mass etc. obtained by our method is almost complete. Applying this method to some cases of twin hull cylinders, it can be concluded that the hydrodynamic interference between twin hull oscillating or in beam seas is not so small and accordingly we must not neglect this interference effect when we want to make a theoretical study of the motions of twin hull ship in waves. If we can obtain the solutions of radiation problem for twin hull cylinder by ur method, then the equations of the motions of twin hull ship in head seas or beam seas can be formulated with the aid of the strip method. The solutions of these equations are shown to be generally in good agreement with the experimental results, but the motions of twin hull ship are different from those of single hull ship and this fact results from the interference effect between twin hull. This effect appears typically in the motions in beam seas and if we do not take the effect into account theory can not predict the motions of twin hull ship.