Because ship motions are heavily affected by wave periods and wave directions, not only wave height but also wave periods and directions must be given in order to obtain the moored ship motions in a harbour. As the first step to obtain the directional spectrum in a harbour, a method for deriving directional spectrums of harbour waves diffracted by two semi-infinite breakwaters is proposed. Moreover, the accuracy of the approximate solution is improved by introducing new velocity potentials neglected in Takayama's method.