To provide a lock protecting device which improves the safety of a door lock by a simple structure, prevents a door from being fraudulently opened with a key alone duplicated with ill will, and prevents the lock from being fraudulently released by picking with the use of a simple bar instrument.

The lock protecting device is constructed by fixing a neck of a knob by using a main body and a cover portion hinged to the main body, and covering a keyhole with the main body or the cover portion. According to the lock protecting device, engagement between the main body and the cover portion is maintained by a numerical electronic key.