Abstract: Two control rings (29a, 29b) are arranged between the cylinder (2) and lock body (1) which are provided with latch grooves (9) receiving that part of the latch (8) which prevents cylinder (2) rotating. The rotor housing has a latch channel (11) formed along the chord of the magnetic body (5) in the rotor (6) and can receive the other part of the latch (8) in a state of magnetic equilibrium determined by the magnets in the key (3) and the magnetic body (5). When a proper key (3) is fitted into the keyhole (10), the latch channel (11) is arranged partly in the rotor (6) housing and partly in the rotor support (4): in this position the cylinder is freely rotatable. When the proper key (3) is removed, the latch channel (11) in the rotor housing is outside the line of action of the latch (8), the latch (8) being arranged in the rotor support (4) and in the latch grooves (9) of the control rings (29a, 29b).