A computer implemented method for calculating an integrated collision risk value, that reflects a danger level felt at an actual site between pairs of vessels, said method executing a process comprising: accumulating AIS information received from vessels at sea at predetermined time intervals, said AIS information associating longitude, latitude, a speed, and a course with a date and time and a vessel identifier; receiving an instruction regarding a period and sea area for calculating, by referring to the accumulated AIS information of each of a first vessel and a second vessel, a base risk value related to a collision between the first vessel and the second vessel at each time point in the instructed period by a predetermined method; extracting a section in which the base risk value exceeds a predetermined threshold; calculating a time point at which the base risk value reaches a local maximum in the extracted section; specifying a time range before or before and after said time point at which the base risk value reaches the local maximum; extracting, from the accumulated AIS information, for each of the first vessel and the second vessel, an amount of change in orientation as an action pattern of each of the first vessel and the second vessel in the specified time range, extracting, from the accumulated AIS information, for each of the first vessel and the second vessel, an amount of change in speed as an action pattern of each of the first vessel and the second vessel in the specified time range; calculating for each of the first vessel and the second vessel, a moving average of absolute values of amounts of change in orientation in the specified time range as an index information A, and a moving average of absolute values of amounts of change in speed in the specified time range, as an index information B, as a degree of an action pattern of each of the first vessel and the second vessel; estimating, for each of the first vessel and the second vessel, a vessel that performs navigation to avoid a collision by using the degree of the action pattern and calculating, as an index information C, the number of vessels performing avoidance maneuvering; and correcting the base risk value into the integrated collision risk value, based on the base risk value, index information A to C, and a weight set for each of the plurality of index information. accumulating AIS information received from vessels at sea at predetermined time intervals, said AIS information associating longitude, latitude, a speed, and a course with a date and time and a vessel identifier; receiving an instruction regarding a period and sea area for calculating, by referring to the accumulated AIS information of each of a first vessel and a second vessel, a base risk value related to a collision between the first vessel and the second vessel at each time point in the instructed period by a predetermined method; extracting a section in which the base risk value exceeds a predetermined threshold; calculating a time point at which the base risk value reaches a local maximum in the extracted section; specifying a time range before or before and after said time point at which the base risk value reaches the local maximum; extracting, from the accumulated AIS information, for each of the first vessel and the second vessel, an amount of change in orientation as an action pattern of each of the first vessel and the second vessel in the specified time range, extracting, from the accumulated AIS information, for each of the first vessel and the second vessel, an amount of change in speed as an action pattern of each of the first vessel and the second vessel in the specified time range; calculating for each of the first vessel and the second vessel, a moving average of absolute values of amounts of change in orientation in the specified time range as an index information A, and a moving average of absolute values of amounts of change in speed in the specified time range, as an index information B, as a degree of an action pattern of each of the first vessel and the second vessel; estimating, for each of the first vessel and the second vessel, a vessel that performs navigation to avoid a collision by using the degree of the action pattern and calculating, as an index information C, the number of vessels performing avoidance maneuvering; and correcting the base risk value into the integrated collision risk value, based on the base risk value, index information A to C, and a weight set for each of the plurality of index information.