On May 16, 1968, the northern part of Japan was attacked by a severe earthquake of Magnitude7.8. The port of Ofunato experienced the first remarkable tsunami since the completion of the tsunami breakwater at the bay-mouth in 1967.
　The authors applied, in this report, the method of analysis presented in former reports to this actual tsunami in order to prove the effect of the existing breakwater. Fortunately, tide elevation due to the tsunami was recorded at two stations, the one outisde the breakwater and the other at the innermost point of the harbour.
　The incident tsunami was determined so that the calculated water level elevation might coincide with the actual one at the outside tide guage station.
　After comfirming good agreement of the computed water level variation with the observed one, the same incident tsunami was applied to state before the breakwater construction. The effect of the tsunami breakwater is apparent from the computed results. If no breakwater had been constructed in Ofunato harbour, the highest water level elevation due to the 1968 tsunami would have been increased to almost twice the actual one.