Navigators take action to avoid the collision with other ship according to the situation awareness or movements of it. These collision avoiding actions taking by navigators are chains of actions based on the navigator's information processing system and collision accidents are occurred by navigators' errors which are happened on the navigators' information processing system, the action of other ship, and chains of theses and combined. Authors made the action model for preventing collision based on the navigator's information processing system and investigated navigators' errors which occurred in every step of collision avoiding action. Furthermore we compare navigators' actions which lead to collisions and avoid collisions safely and obtain useful information to prevent collision accidents as follows. ・ Navigators' errors are repeated in the steps of navigators' collision avoiding actions. ・ Navigators' errors are occurred in the step of "Awareness of other ship" or "Recognize other ship's movement". ・ It is important that navigators recognize other ship's movement exactly to avoid the collision. ・ Navigators aware the other ship, recognize the other ship's movement exactly, and estimate and confirm the risk of collision by several means in all cases of avoiding collision safely we investigated.
