It is the most important special characteristic that ships all have to hold the ability to complete their navigations by themselves, and it is defined as "the self-completion ability" in this paper. The purposes of this paper are to investigate the values of varying quantities of ship's self-completion ability on the marine traffic accidents in Japan, and for a premise, to introduce the new function for rating this ability. The new function is composed of the transition ratio from the lower level damage or risk to the higher, and the levels are arranged in a series on the process of traffic accident. The numerical values of this ability are obtained by the statistical data of all the casualties and the marine accidents requiring rescue in the latter half of the twentieth century in Japan. In consequence of these numerical results, this paper points out the average of the self-completion ability at the low level risk is improving after the 1970's but the ability at the higher is not very much change during the past about 50 years, and therefore, it is necessary to improve the ability obstructing transition to the higher level risk.
