PURPOSE: To ensure triangulation even if there are shielding objects, by providing at least three fixed points, which are used for the triangulation, and sequentially selecting arbitrary two fixed points, which can be surveyed, between the fixed points and a moving body.
CONSTITUTION: A situation such as follows is supposed. A shielding object 250 of light such as dump truck enters into a working area 100 of a bulldozer 200 and performs other works. An area E shown by a shaded part is an area, where laser light from a light projector 4 is shielded. When the bulldozer 200 is located at position coordinates C n-2 and C n-1 at times t n-2 and t n-1 , laser lights, which are projected from light projectors 2, 3 and 5 other than the light projector 4, are judged to be received. For example, triangulation is performed in the combination of e.g., the projectors 2 and 3. The predicted position coordinate C n ' at the time t n is operated based on the result.