To inform a ship operator or the like in an easily understandable way of the existence of a ship having the chance of collision and the degree of emergency in terms of time.

Vertex A of a triangle is determined based on a time TC it takes for a ship to reach a point which most approaches a predicted collision point A in a current course of the ship and a distance S from that point to the predicted collision point. Apex C of the triangle is determined by obtaining a time TS it takes to reach a point in the course of the ship at which a distance between that point and the predicted collision point becomes a predetermined distance (1 nautical mile). Based on the right triangle ABC, the ship operator or the like is informed of the degree of danger of the ship with regard to the predicted collision point.