PURPOSE: To prevent a dishonest unlocking by making it possible to insert a key plate completely only when notches continuously passing through window sections of a slide plate correspond with positions of each of tumbler projections in a stage directly before the key plate is completely inserted.

CONSTITUTION: Plate-like tumbers 11 - 16 are inserted into radial sliding grooves of a rotor 2, and a slide plate 4 is loaded to the peripheral surface. After that, when a key plate 20 is inserted to a stage directly before the complete insertion, the bumblers 11 - 16 maintain specific positions by keyways of the plate 20, and each of tumbler projections 12a - 16a corresponds with positions of notches 81 - 85 between window sections of the plate 4. When the plate 20 is pushed forward, the plate 4 is such that the notches 81 - 85 pass through the projections 12a - 16a to make it possible to advance, and it is completely inserted into a lock 10 to allow the rotation of the rotor 2. Accordingly, the number of wrong keys can be substantially increased to prevent a dishonest unlocking.