PURPOSE: To prevent a burglary by providing a sensor detecting the insertion of a foreign matter into a lock, and providing a stopping mechanism for making unlocking impossible through the induction of the sensor unless a specific key is inserted and an alarm device giving an alarm through a signal from the sensor.

CONSTITUTION: A door cylinder 100 is constituted of an outer cylinder 1, an inner cylinder 2 and a key insertion open 3. Holes 4 and 5 are respectively bored in the cylinders 1 and 2, an X shaped through-hole is formed in a state to insert no key, and a sensor 6 is mounted. A recession 50 is provided to the surface of the inner cylinder 2 from the outside of the outer cylinder 1, and a stopping mechanism 70 of a stopping piece 71 capable of loading and unloading and a driving section 72 inserting the stopping piece into the recession 50 is provided to the recession 50. After that, when a foreign matter is inserted into the cylinder, the sensor 6 senses, the driving section 72 is operated by the signal, the stopping piece 71 is inserted into the recession 50, the movement of the inner cylinder 2 is provided to check unlocking, and the alarm system is operated to give an alarm.