A theoretical derivation, according to the interpolation slender body theory, is presented for the diffraction problem of a fixed ship, moving with forward velocity in regular head waves. Assuming that U = O (ε^1/2), ω₀= O (ε^{-1/4+ b} ) (0< b <1/4), the boundary value problem is linearized in the outer and inner regions respectively. The inner solution, governed by two-dimensional Laplace equation, involves an interaction effect between the steady forward motion and the unsteady incident wave motion. A similar matching procedure, as for the radiation problem, is used to determine the unknown source strength and the homogeneous component in the inner solution. Computations and measurements are carried out for the wave exciting force of a slender ship model, with cross sections defined by Lewis forms. Comparing the results, it appears that the present method gives good predictions, including the interactions between sections along the length.