Abstract: The locking device consists of a key (2) and a lock cylinder (1), with a cylinder core (5) which is rotatable in a casing (3) and has an axially extending channel (16) for insertion of a key (2) which passes through plate-shaped tumblers (7) arranged axially in succession so as to be displaceable in the cylinder core (5) and, with bit stages provided on the key shank (13), locates these tumblers (7) in a position releasing the rotation of the cylinder core (5). The locating displacement movement of the tumbler plates (7) is designed as a rotational movement around the axis of the key shank (13), and its range is preferably limited by stops (21, 22).