The forced oscillation method is generally used to examine hydrodynamic derivative coefficients of the equations of ship motions. This method is to analyse the ship motions caused by known mechanical force or to analyse the hydrodynamic reactions caused by known displacement of ship in still water. In this paper, however, the authors propose new forced oscillation method which uses water waves instead of known mechanical forces. This method has two merits. One is that hydrodynamic derivative coefficients can be obtained without using the apparatus for forced oscillation method, and the other is that the investigation of hydrodynamic coefficients from new aspect is possible, because they are obtainable in the field where water waves exist. The results obtained by applying this new technique to longitudinal motions almost coincided with ones by conventional forced oscillation method, and its effectiveness was shown. Moreover, the authors were able to examine the main terms of sway and roll in beam wave obtained by the above-mentioned technique expanded to lateral motions.