A short-term planned route from a start position that is a ship position at a first time up to an end position that is the ship position at a second time is designed based on a planned route, an estimated encounter marine phenomenon information, operation performance information measured on a ship, and a hull motion model of the ship, the estimated encounter marine phenomenon information being measured on the ship based on actually encountered marine phenomenon information. The short-term planned route makes a first evaluation function optimal, the first evaluation function containing: an index indicating an influence of a fluctuation portion between a planned position that is a ship position planned at the second time on the planned route and the end position; a fuel consumption index when the ship sails along the short-term planned route; and a safety index when the ship sails along the short-term planned route.