A method includes: extracting Time to Closest Point of Approach included in a predetermined time from “risk value information” that stores a “Closest Point of Approach”, the “Time to Closest Point of Approach” and a “risk value” for “a first vessel and a second vessel”, the risk value being a value indicating a possibility of collision between the first vessel and the second vessel at the Closest Point of Approach and the Time to Closest Point of Approach; acquiring the Closest. Point of Approach and the risk value corresponding to the extracted Time to Closest Point of Approach from the risk value information; determining to which sea area the acquired Closest. Point of Approach belongs to; and executing calculation processing that includes calculating a sum of risk values corresponding to the Closest Point of Approach for each of sea areas to which the determined Closest Point of Approach belongs.