In Japan, huge volume of cargo is transported by ships. Therefore, volume of maritime traffic comes enormous. More than nine million ships call major Japanese ports in one year. 　It is very important to estimate of maritime traffic flow, for planning facilities inside the port and navigation channel outside the port. However, the statistics on maritime traffic flow is not available now in Japan. Though various methods have been tried such like Rader observation and analizing of aerophoto, maritime traffic flow all over the country has not been estimated before this study. 　In this study, a method to estimate maritime traffic flow from the statistics of cargo flow is developed; and trip of ships among major Japanese ports is estimated with the statistics on cargo flow. From 2.7 million data, 188x188 O-D matrix of maritime traffic flow is established for eachvessel category.