The wave observation on the Japanese coast through the Nationwide Ocean Wave information network for Ports and HArbourS (NOWPHAS) has been conducted since 1970. The water depth and surrounding bathymetry of the observation site, equipment and data processing procedures changed at some locations during these four decades. This study, therefore, reviewed the change in the quality of the NOWPHAS data and then verified the long-term trend in the seasonal wave characteristics at 24 locations as well as the correlation between the wave characteristics and climatic indexes (SST, SO, AO, PDO and NPI) at 10 locations. The major conclusions are as follows:
　1) The annual mean significant wave height increased at Ainoshima, Mutsuogawara, Hitachinaka and Nakagusukuwan and decreased at Habu. The seasonal mean significant wave heights increased at some locations.
　2) The annual mean and the seasonal mean significant wave periods increased at many locations where extra-tropical cyclones are major inputs on waves rather than typhoons.
　3) The mean wave energy flux increased at some locations mainly on the Sea of Japan and the Pacific Ocean coast of the southern region of Japan.
　4) The annual-largest significant wave height increased at Sakata, Naze, Mutsuogawara and Hachinohe, the winter-largest significant wave height at some locations on the Sea of Japan coast, and the autumn-largest significant wave height at some locations on the Pacific coast of the Tohoku District. A significant trend is not significant in the largest significant wave periods.
　5) The correlation of the mean significant wave height with the climatic indexes is low except SST.