In the preceding reports, the calculating way called wave motion method and approximate one-dimensional method were established and they were applied to Ofunato-bay. Since then, the effect of breakwaters against long-period waves has been investigated by numerical computation with a digital computer, for Nigata Port, Matsushima-bay and Nagoya port.
　In this report, the results of the computations are compared with those of experiments made in a simple L-shaped model basin in order to investigate how these theories agree well with a real phenomenon. The results are as follows.
　1. Results obtained by wave motion method show a good agreement with those of experiments.
　2. Approximate one-dimensional method gives a good result only in case of without breakwaters.