PURPOSE: To improve the responsiveness and the precision of a system where position measurement is performed on a basis of position data of a radio navigation device such as a loran, by monitoring the time delay of a position measuring filter by another filter having the same coefficient as the position measuring filter and correcting the output of the position measuring filter on a basis of the monitor result.

CONSTITUTION: The output of a dead-reckoning navigation device 3 is supplied to a filter 2' having the same coefficient as a position measuring filter 2 and is sent to a subtracting circuit 4 also, and the difference between the output of the dead-reckoning navigation device and the output of the filter 2' is obtained in the subtracting circuit 4. Since filters 2 and 2' have the same coefficient and the output of the dead-reckoning navigation device 3 is the position data which has no variance and has a comparatively high precision in a short term unlike the output of loran, the output of the subtracting circuit 4 is equal to a position variation corresponding to the time delay of the filter 2', namely, a position variation corresponding to the time delay of the filter 2. That is, the filter 2' is used to monitor the time delay of the position measuring filter 2, and the output of the filter 2 is corrected on a basis of this monitor result.