PURPOSE: To reduce the number of detectors and to perform an exact detection by detecting the objects, the sizes of which are made distinct from each other so that the objects are made a combination corresponding to the respective detection position on the traveling line, by means of detectors loaded on the moving body and by detecting the location of moving body.

CONSTITUTION: The detectors 7W9 are loaded on the moving body 6, and these are arranged so that the detector 7 is made a reference one and the distances between the detector 7 and the ones 8, 9 are a and b respectively. In the reaction plate 10 of respective detection positions of the traveling line 1, the pocket hole 2, fixed point 3, inversion hole 4 and overrun hole 5 as the objects to be detected are provided. Also two holes the distance between which is a are provided in the reaction plate at the hole 2, two holes the distance between which is b for the fixed point 3, two holes the distance between which is (b-a) for the hole 4 and three holes 11W13 for the hole 5 are done respectively. The holes 11W13 are arranged so that the hole 11 is made a reference point and the distances between the hole 11 and the holes 12, 13 are a and b respectively, the hole 12 is different from other holes 11, 13 in size. In this way, it can be avoided that the location of hole 5 is decided as that of the fixed point 3, the number of detectors can be reduced.