PURPOSE: To prevent opening of a lock by illegitimate means by a method wherein the distance between a projection of a specific tumbler and a side wall of a groove in a holder is made longer than the distance between the projections of other tumblers and the side wall of the groove in the holder, and the other tumblers are placed in the foreground of the specific tumbler.

CONSTITUTION: A specific tumbler is provided to the inner side of other tumblers 8. The specific tumbler is so made that the distance H between a locking member 10b of the specific tumbler and a side wall 7d of a groove 7a provided to a holder 7 becomes longer than the distance (h) between locking members 8b of the other tumblers 8 and the side wall 7d. Thereby an attempt of forcible lowering of the specific tumbler under a condition where the tumblers 8 are lowered resisting against impetus-giving force of a spring 4 and are forcibly mounted on the inner peripheral surface 7b of the holder 7 is prevented with an instrument employed for insertion into a key hole 8d for the tumblers 8 interrupted by a key-contact face 8e of the key hole 8d.