PURPOSE: To surely prevent unlawful unlock, and in addition improve durability of parts related to a tumbler in particular.

CONSTITUTION: When a key rotor 30 is operated to rotationally move by inserting a key except the regular key, a driver, or the like, a sleeve 26 is rotationally moved together with the key rotor 30. Then, a movable bar 29 is pushed and moved in the arrow mark C direction by means of a cam part 24 provided on the inner circumferential part of a rotor case 21, a lock lever 38 is moved to a diengaging position in the rear part and disengaged from the key rotor 30, and hence the lock lever 38 is not rotationally moved so as not to be unlocked. At this time, the sleeve 26 is not moved in the axial direction, the positions of a tumbler and the sleeve 26 are not changed, and hence the tumbler and the sleeve 26 are hardly injured.