Campaigns for preventing marine casualty in restricted visibility are carried out every year, but these type of accidents to continue occur. Awareness campaigns suggest using radar suitably, observing the conduct of vessels in restricted visibility, among other precautions. However, other studies have pointed out that some navigators improperly understood how to apply rule in restricted visibility and how to use vector modes on the radar screen.When taking these situations into account, this study investigates how navigators are applying rule in restricted visibility and their understanding of vector mode. The study was conducted by questionnaires, and the results suggest the following: (1) Many navigators confuse conduct of vessels in sight of one another and conduct of vessels in restricted visibility.(2) Some navigators understanding of vector mode is dubious. In the final part of paper, measures for preventing marine casualties are discussed.