A lock device having a lock cylinder and a flat key having a control gate. The lock cylinder has a cylinder housing and a cylinder core rotationally supported in the cylinder housing and a plurality of plate-shaped tumblers that are supported in the cylinder core so as to be radially slidable with respect to the rotational axis of the cylinder core and which each have a key opening and are at least partially pushed by the spring load into a locking channel of the cylinder housing in order to lock the lock cylinder when the flat key is removed and which are slid into an unlocking position by the control cam guided in the control gate when the key is inserted. At least one stop is associated with each of the tumblers in order to limit the sliding path to conceal the unlocking position.