When a ship is travelling in a following sea, the frequency of encounter is reduced to be very low. In this case, however, the so-called strip theory can not evaluate the wave exciting force correctly, because it does not take the lift component into consideration. In the present paper, a theoretical method is described for calculating the diffraction force. It is based on the generalized unsteady lifting-surface theory; thus the lift force is taken into account, and the free-surface effects in the high speed as well as low frequency range can be also incorporated. Numerical computations using the present theory are carried out, and then the results are represented as a function of U / C , where U denotes ship speed and C phase velocity of a wave. Furthermore, corresponding experiments are conducted for a flat plate and a Wigley ship model. Through the comparison between computed and measured values, it is confirmed that the effect of encounter frequency on the diffraction force in following seas can be explained in terms of the present theory.