The knowledge of the directional spectra for surface waves is essential to the problem of the wave generating mechanism by wind, and to the related problem of wave forecasting. It is important to the understanding of coastal phenomena, and consequently to the design of coastal works.
　The author proposed a new method of determining the directional spectra of sea waves using the record of a wave meter and the record of a wave direction meter which can record X and Y component wave force acting on a bottom mounted sphere.
　The accuracy of the method was investigated by means of simulated wave properties with directional spectra.
　One set of actural data was analyzed by the method proposed herein, to obtain the approximate directional spectra. In addition, the value of CD and CM in the conventional wave force formula for the sphere were obtained as a function of component wave frequency.