PURPOSE: To facilitate the updating of a key and the operation of the reverse master key system and improve the security by providing a measuring device having a light emitting diode and an optical fiber and detecting the arrangement of key ridges of the inserted key.

CONSTITUTION: A measuring device 1 constituted of a fixed position detecting sensor 5 and a key ridge height measuring sensor 2 is provided, the arrangement state of the height of key ridges 81W85 of the inserted key 8 is detected. This key ridge height measuring sensor 2 is constituted of a light projecting section 3 made of a light emitting diode or the like and a light receiving section 4 made of an optical fiber cable. The detected key ridge arrangement state is sent to a judging device, if it coincides with the previously registered one, an unlocking signal is issued. The key 8 can be simply discarded or updated by the registration change to the judging device, the master key system and the reverse master key system or the like can be freely operated.