PURPOSE: To secure the safe navigation of a ship by confirming the position of the ship by transmitting and receiving acoustic wave pulses in a sea area of poor visibility.

CONSTITUTION: In a measuring station, a distance measuring means 5a measures the distance to a ship to be measured from the point of time when an interrogating pulse is transmitted from an interrogating pulse generating means 10 to the point of time when the response pulse from the ship is received and an azimuth measuring means 6a measures the azimuth to the ship when the response pulse is received. On the ship, a distance measuring means 5 measures the distance to the measuring station, distance between the measuring station and another ship to be measured, and distance between its own ship and another ship from the time spent until confirmation pulses transmitted from the confirmation pulse generating means 11 of the measuring station to its own ship and another ship are received and the time spent until a response pulse transmitted from the response signal generator 4 of another ship is received. In addition, the azimuth measuring means 6 measures the azimuths to the measuring station and another ship when the confirmation pulses and response pulse are received.