The effect of breakwaters against long-period waves, such as tsunami, is investigated in this report by numerical computation and its analysis, continuously to the preceding report published in 1962.
　One of the main items to be studied is the phenomenon of resonance. However, as a tsunami usually attacks initially calm basin, the problem should be treated not only as a stationary state, but also as a transient state.
　In principle, the calculation is to solve the equations of motion and the equation of continuity for two-dimensional long wave of small amplitude under certain initial and boundary conditions. The basic equations are transformed into difference equations and numerical computations are executed by electronic computors.
　The incident wave profile is assumed to be a train of sinusoidal waves with a constant amplitude and period. These waves invade through a breakwater gap or a bay-mouth into a restricted water basin where the water is originally at rest.
　There are various factors which will affect the phenomena. The investigation was begun with simplified cases so as not to lose the way and was put forward step by step examining the influence of each factor.
　In the preceding report, the behaviour of waves through a breakwater gap situated in a semi-infinite channel with a rectangular cross section of uniform depth was studied first. The wave invasion into a rectangular bay was also treated. After analyzing the result of these two-dimensional calculations, an approximate one-dimensional method was proposed.
　The influence of the shape of bay and the reflexion from the innermost boundary inside the basin are discussed in the present report. In the last chapter of this report, the calculating method is applied to Ofunato-bay, where a new anti-tsunami breakwater is under construction, taking into account the actual topographic condition and water depth distribution.
　Although there remain some items which should be further investigated, the computation method established in the reports is widely applicable and is able to give us fairly definite informations necessary for planning and designing of anti-tsunami breakwaters. The approximate one-dimensional method is also useful for getting general resonance characteristics of the basin.