Abstract: A lock cylinder placed in an optional lock (A) positioned on an optional door (B) and consisting of a housing (1), preferably having standard external dimensions, shape and type, of an optional plug (2) with a cam (18) that act on the mechanism of the lock (A), of a knob (5) for manual unlocking and locking of the lock (A) from the side of the room locked by the lock (A) is positioned on the front side of the insert on that side of the door (B), from which a room to be locked is entered, whereas the cylinder is provided with a protecting element, preferably in the form of a protecting cover (13) allowing authorized access to the slot in the pin (2) for a key, whereby said protecting element is entirely fit in the seat (40) arranged in a security escutcheon (39) positioned on the door (B).