PURPOSE: To permit key information to be read out electrically by a method in which a key is inserted into the key hole of a reader to detect the shape of the carved groove of key by many key information sensors set at the shortest carved groove intervals of key.

CONSTITUTION: Luminous elements 2aW7a and 2bW7b are set on upper and lower stages at the shortest carved groove intervals of key K on the corresponding portion of one side of key K to be inserted from a key hole 1a formed in the center in the longitudinal direction of the reader body 1. Many optical fibers 16aW16c and 17aW17c are also set at the minimum step difference intervals of the carved groove of the key K on the opposite side of the elements 2aW7a and 2bW7b. Phototransistors 18aW18c and 19aW19c as photodetectors 20 are optically connected to the ends of these elements. The shape of the key K inserted into the key hole 1a is optically detected to read out it electrically by the photodetectors 20.