PURPOSE: To prevent wrong unlocking of key by a method in which when a door key or the other is inserted into a cylinder lock, a coupling piece is fitted into a recession leading from the outside of an outer cylinder to the surface of an inner cylinder to interrupt the rotation of the inner cylinder.

CONSTITUTION: A sensor 6 to detect the face that a door key 200 or other is inserted into a cylinder lock is provided, and a recession 50 leading from the outside of an outer cylinder 1 to the surface of an inner cylinder 2 is also provided. A coupling mechanism 70 with an inward energized coupling piece 71 to fit into or come out the recession 50 is provided on the corresponding place of the recession 50. When a detection signal is sent out from the sensor 6, the drive part 72 of the mechanism 70 is operated to fit the piece 71 into the recession 50 and also to prevent the rotation and unlocking of the inner cylinder 2 by sending an alarm. Also, by setting the sensor 6 to OFF previously, proper unlocking can be permitted.