To provide a door key system for rapidly taking the action of a safety measure in emergency such that a client key is lost.

During construction, a cylinder lock 20 is locked and unlocked by a working key 21, and after the construction, an interior structure is modified by inserting the client key 31 into the cylinder key 20 to make the same structure as the interior structure of a cylinder lock 30. Thereby the working key 21 cannot be used for the cylinder lock 20, and the cylinder locks 20 and 30 can be locked and unlocked by the client key 31. Furthermore, in emergency, the interior structure of the cylinder lock 30 is modified by inserting an emergency key 41 into the cylinder lock 30 which cannot be locked and unlocked by the client key 31, and the cylinder lock 30 can be locked and unlocked by the emergency key 41.