PURPOSE: To reduce the trouble of theft by making the key release difficult by forming an inclined projection part on the side well of the groove of a holder and forming the projection part of the lock part of an added tumbler in projection state in comparison with other parts.

CONSTITUTION: An inclined projection part 7c is formed on the side wall 7b of the groove 7a of a holder 7, and an another tumbler 9 separated from each tumbler 8 is installed. The projection part 9b of the lock part 9a of the tumbler 9 is formed in projection from those of others. Accordingly, even if any tumbler 8 is pushed down by using a jig, etc., the projection part 9b contacts the inclined projection part 7c of the holder 7 when another tumbler 9 is pushed down. A rotor 2 is turned to the other side when riding over the projection part 7c, and the engagement between the lock part 8b of other tumbler 8 and the holder 7 is disengaged, and the tumbler 8 projects into the groove 7a, and the rotor 2 is set into turn-disabled state. Further, the position relation between the tumbler 8c and a key insertion port is allowed to coincide, and the visual confirmation for the key top arrangement is made difficult. Accordingly, the trouble of a theft can be remarkably reduced.