Abstract: Security device for lock cylinders, which consist of a rotor (1) which rotates in the cavity of a stator (2), which locks use true keys (4) in which the combination for opening is by means of peaks and valleys of different sizes which operate in conjunction with respective pairs of pins (5-6), which, when accommodated in the key opening position, consist of a rotor pin (5) and a stator pin (6) which is loaded by a spring (7); the rotor (1) incorporates the channel (8) for insertion of the true key (4) in which there is provided a usual virtual line (9) according to which there is longitudinal alignment of the tips of the rotor pins (5), in the state in which the key is not inserted; characterised in that at least one of the rotor pins (5), which shall be known as the security pin (10), has some end means which prevent the remainder from reaching the usual virtual line (9) in the state in which the key is not inserted or the lock is empty, and its advance reaches only a virtual security line (9B) which is shor