PURPOSE: To enable the detecting of the position of a vehicle even when only two satellites exist from which satellite radio waves can be received at the vehicle.

CONSTITUTION: A position measuring section 8b of a GPS receiver 5 performs a one-dimensional position measurement using the newest alititude data determined to determine a line of existence of a vehicle on a fixed spherical surface by a value with the alititude known when the number of satellites to be captured with a demodulation section 7 is down to only two to disable three- dimensional and two-dimensional position measurements and outputs the results to a map matching section 13. The map matching section 13 calculates an intersection between a road where the vehicle runs or a road linked to the road and a line of existence of the vehicle near the position of the vehicle using a road data of a CD-ROM 1 to determine the new position of the vehicle. A map image drawing section 12 controls a reading control section 15 based on the position of the vehicle to read out a map by one portion of screen centered on the position of the vehicle from among maps drawn in a video RAM 14. A vehicle position mark is synthesized with a synthesizing section 17 centered on the map read out to be shown on a CRT display device 3.