Abstract: A mechanism which may be added to existing conventional type cylindrical door locks at the time of manufacture thereof or to such locks already in the field with just a slight modification of the existing lock. The structure of the added security mechanism blocks the throw of the latch bolt and at the same time blocks the retractor to positively prevent opening of the lock by key, turning of the knob, or tampering with the latch. The security device is tamperproof as it is inserted into the interior of the cylindrical lock and can only be disengaged to allow entry from inside the building structure. The lock cannot be opened from the exterior of the door when the security device is engaged, by any conventional method or tampering.