It is important to collect many wave data in order to know long term distribution of a ship response caused by wave loads. Japanese Meteorological Agency (JMA) has been collecting observed wave data from many ships since 1976. The amount of data is enormous and sufficient to exaimine the statistical characteristics of wave parameters. Wave data collected from merchant ships consists of some items, which are direction, period, and height of significant wave for wind wave and swell. There are some human errors in observed data because many data are obtained by visual observation. And the accuracy of some collected wave data is not so good. This paper examines the characteristics of visually observed data obtained from merchant ships by using two methods compared with weather ships of JMA. One is the comparison method between probability distributions in the same region and the other is the comparison method between near ships at the same time.