Abstract: A protective device for key-operated door locks, comprises a housing mountable to the door over the keyhole of a lock mounted within the door; a guard plate movable to a blocking position to block access to the keyhole, or to an unblocking position to permit access to the keyhole; and a rotary knob rotatable to a plurality of positions and having markings thereon indicating its position. A transmission mechanism including a plurality of notched code discs is provided between the rotary knob and the guard plate for moving the guard plate from its locking position to its unlocking position when the code discs have been positioned to predetermined coded positions such that further rotation of the knob moves the guard plate from its blocking position to its unblocking position.