The computation of wave diffraction is very important to determine the alignment of breakwaters properly. Considering the irregularity of sea waves, Nagai has presented the computation method as for directional spectral waves. His method is a powerful tool in the resolution of practical diffraction problems, but it is not applicable to the following cases:
　1) For cushion type breakwaters with arbitrary reflection coefficients
　2) When the incident waves are reflected by the rear side of a breakwater
　3) When the diffracted waves by a breakwater are again diffracted by the other one.
　In this paper, new practical computing method is presented, which resolves the remaining problems in Nagai's method. Then, the method is applicable to any type
of two breakwaters, either rigid or cushion type. As for regular waves, the method is verified in the comparison with the numerical analysis of wave propagation, which is presented by Ito and Tanimoto. In the results of comparison, it has become clear that the method is available for the computation of diffracted waves. Furthermore, the method has been verified by the experiments for the diffraction of the unidirectional irregular waves, and confirmed to be applicable to practical problems within an acceptable accuracy.