PURPOSE: To prevent a malfunction by an improper key by forming a keyway to one surface of a key in the longitudinal direction and forming a projection for positioning capable of being fitted into the keyway to the inner peripheral section of a tumbler.

CONSTITUTION: Meandering keyways 102 are formed to the surface and rear of a key body 101 respectively in a key 100. When an improper key is inserted, the projections 48b-41b for positioning of tumblers 48-41 are slid successively into the keyways, and the tumblers 48-41 are slid in succession to the insides. Notch sections 41c-48c are not arranged on the same straight line at that time, and the projections 53 of a side bar 50 do not fall into the notch sections 41c-48c. Consequently, left and right levers 61, 62 cannot be rotated, and pawl sections 61c, 62c for engagement are not opened outward, thus preventing the engagement of the pawl sections 61c, 62c for engagement with the projections 71, 72 of a rear rotor 70. Accordingly, only the main rotor 20 is turned even when the key 100 is revolved, the rear rotor 70, a position of which is controlled by a coil spring 80, is not rotated, and the positions of the tumblers 41-48 are regulated forcibly. thus preventing a malfunction.