PURPOSE: To improve a position measuring accuracy by converting an output of a biaxial electromagnetic log to a local horizontal surface by a pitch angle and a roll angle of a vessel, setting it as a horizontal speed, and deriving the passage and position of the vessel from said output and an output of a gyrocompass.

CONSTITUTION: Outputs of a biaxial electromagnetic log 1, a pitch angle measuring instrument 5 for measuring a pitch angle of a vessel, and a roll angle measuring instrument 6 for measuring a roll angle of the vessel are inputted to a horizontal speed operating circuit 7, and a horizontal speed component which has converted a bow and stern axial direction water speed and a pitch shaft axial direction water speed being outputs of the biaxial electromagnetic log 1, to a local horizontal surface, based on the pitch angle and the roll angle, that is, a bow and stern axial direction horizontal speed and a pitch axial direction horizontal speed are calculated. Subsequently, a passage and position operating circuit 8 inputs an azimuth being an output of the horizontal speed operating circuit 7 and an output of a gyrocompass 2, and the passage and position of a vessel are derived.