PURPOSE: To detect he reverse connection of the respective optical fibers of optical fiber cables.

CONSTITUTION: The ends of the respective optical fibers 12 of the optical fiber cables 11 are respectively held by a connector ferrule 13. The light rays from the respective optical fibers 12 of the connector ferrule 13 are detected by a photodetector 14. The light rays of the respective optical fibers 12 passing between the connector ferrule 13 and the photodetector 14 are shielded one by one by a light shielding mechanism 15. Then, the directivity of the light rays of the respective optical fibers is detected by checking the light shielding directions of the light rays of the respective optical fibers by the light shielding mechanism 15 and, therefore, the reverse connection of the respective optical fibers of the optical fiber cables 11 is detected.