National marine accident investigation authorities conducted an analysis of causal factors by using the Swiss cheese model as theoretical background after collecting evidence with the SHEL model in accordance with the Casualty Investigation Code adopted by the International Maritime Organization. The causal factors of serious marine accidents were classified according to each element of the SHEL model. The occurrence of accidents and each element of SHEL have a cause and effect relationship; therefore, the number of vessels involved in a serious marine accident is regarded as the dependent or y variable, and each element of the SHEL model as an independent or x variable. The most influential element of the SHEL model on the number of vessels involved in accidents was software, and then peripheral liveware. Operators and organizations are able to see contributing factors which helped holes open and then lead their vessels to accidents.