To notify, when a ship at sea may encounter the appearance on or crossing of its expected course by a hazardous object or another ship, whether the expected course of voyage is safe or not by capturing, by a radar through such a system as the automatic radar plotting aids (ARPA) or the automatic identification system (AIS), information on a hazardous object and the position and course vector of another ship, calculating the closest point of approach (CPA) and the time to CPA (TCPA) to the expected course, and displaying a hazard symbol.

By causing an arithmetic unit to calculate CPA and TCPA at each point on an expected course of voyage by using searched object pictures, information on a subject ship, information on other ships and information on the expected course obtained by a radar antenna part and judging whether an alarm is needed according to the result of calculation, a hazard symbol according to the judgment is displayed on the screen of a display unit.