PURPOSE: To obtain an initial coordinate value setting method of an inertia detecting means mounted in a moving body which makes it possible to accurately set in a short time an initial coordinate value of the inertia detecting means of the moving body moving from a moving base such as a vessel moving within a navigation coordinate system and to easily determine the relationship between a coordinate system of the inertia device of the moving body and a coordinate system of the moving base.

CONSTITUTION: A moving body 16 having an inertia detecting means 18 for detecting the physical amount and the inertial force necessary for the inertial navigation is set to a moving base 10. When the moving body 16 is moved from the moving base 10 within a reference navigation coordinate system, the initial moving direction of the moving body 16 is detected and stored from a measuring value of an inertia detecting means of the moving base 10. Moreover, the initial acceleration of the moving body 16 is detected by the inertia detecting means 18 of the moving body 16. When the direction of a combined acceleration obtained from the detected acceleration is not coincident with the stored initial direction, the angular shift to the reference navigation coordinate is obtained from the combined acceleration and each measuring value of the axial acceleration of the inertia detecting means 18, and the initial coordinate value is set after the shifted angle is corrected.