PURPOSE: To make sure that the tracking point will not be off from the range of a signal that is the object of integration even if the speed of ship is accelerated by seeking the speed of integrated average start timing from the speed of an actual ship.

CONSTITUTION: The memory content of counter TM and TS1WTSn in which integrated average start timing is stored are changed based on the memory content of a speed registor MV, and the course from a gyrocompass 12 and the speed relative to water of a log 11 are respectively input. From those data, station position data, an estimated speed of integrated average start timing is sought. With this arrangement the tracking point is not lost even in a weak field strength area tracing with high accuracy can be made.