PURPOSE: To detect a position of a moving body over a long distance with the use of an optical distance-measuring device on the moving body.

CONSTITUTION: A reflecting plate 18 reflecting light radiated from an optical distance-measuring device 12 on a moving body 10 is arranged sideways of a moving path P to be inclined to the moving path P of the moving body 10. The optical distance-measuring device 12 is set with an inclination of an angle θother than right angles to the moving path P so that the light radiated from the measuring device 12 hits a reflecting face 16 of the reflecting plate 18 approximately at right angles. The optical distance-measuring device 12 measures a distance between the moving body 10 and reflecting plate 18 from a time (reflecting time) after the light is radiated before the reflected light is returned. A moving distance of the moving body 10 is detected from a change of the distance and the angle 9.