PURPOSE: To enable the current position of a mobile body to be easily and accurately measured via a simple arithmetic process by measuring the current position via self-contained inertia navigation, when a current position measurement condition with GPS navigation is not satisfied.

CONSTITUTION: When transmission data is not received from an earth satellite during the travel of a mobile body from an initial set position (reference position), the current position of the mobile body is obtained by a self-contained inertia navigation measurement section 5, and shown in a display section 12. In this case, when transmission data from three or more satellites satisfying a measurement condition with GPS navigation is received by a GPS receiving section 1, the current position of the mobile body is obtained by GPS navigation measurement section 2 and shown in the display section 12. Thereafter, the current position is continuously calculated by the self-contained inertia navigation measurement section 5, and shown in the display section 12, using the current position obtained with the GPS navigation as an initial set position. In this case, when transmission data from one or two earth satellites is received by the GSP receiving section 1, a complex arithmetic section 8 obtains the current position through arithmetic processing, combining the GPS and self-contained navigations. This current position is used as an initial set position for continuing to calculate the current position of the mobile body with the aforesaid section 5 and display the position in the display section 12 through the self-contained navigation.