Computing Programs of the diffaction of diffracted waves, and irregular wave diffraction due to the gap of breakwater have already been developed in the Port and Harbour Research Institute, and have been applied to practical problems. However, the utilization of those programs be restricted by some few conditions. Lately, new computation program has been developed by Takayama et al. In this report, the outline of the new computation is discribed. Then the new program was compared with the previous ones in the computed result, and their coincidence is confermed in comparison with their computed results which are carried out in valid cases for the previous program.
　The new program has already been applied to practical problems such as the designs of breakwater alignments and extensions. However, since the relation between the dividing number of wave spectrum and computing accuracy had not been sufficiently discussed.
　In this report, and approximate formula for the determination of the dividing number is alsoproposed, referring to the detail investigation of the relation between the computing accuracy and the number.