This paper introdeces several interesting facts obtained from the 20-year NOWPHAS (Nationwide Ocean Wave information network for Ports and HArbourS) wave data between 1970 and 1989.
　By using the statistical 't' distribution theory, the quantities of the three types of time fluctuations (long term, seasonal, and short term ones) of the observed wave heights are evaluated.
　From the view points of the short term fluctuation, particular high and low wave days in the year are picked up, by way of the three diferent methods. It is also clarifyed statistically that during several days around July 20th, the Memorial Day of Ocean in Japan, waves are relatively smaller at the both Japan Sea and Pacific Ocean Coasts.