The three-dimensional boundary value problem for the unsteady motion of a ship which is translating and oscillating on the free surface of a deep water formulated. Under the assumption that the forward speed is small and order of $varepsilon$, all formulations are made up to the first order of $varepsilon$. For the numerical calculation, the three-dimensional source distribution method is applied, and the triangular elements are used to represent the hull surface. The results for the added mass and the damping coefficient for Series 60, $C_B=0.7$ at Fn=0 and Fn=0.2 shows good agreements with those of Inglis, Chang, and Inglis and Price.