PURPOSE: To prevent dishonest unlocking by inserting a tumbler lock plate having one side opposed to notched teeth and the other side opposed to an interlocking pin into an open-hole of a rotor, and interlocking the interlocking pin with the notched teeth against turning force to the rotor.

CONSTITUTION: A specific number of sliding slots 3 are bored in a rotor 2 to insert a tumbler 4, an interlocking slot 1a is formed lengthwise on the circumferential surface of a cylinder 1, and the tumbler 4 is energized in the radial direction. An open-hole 2a passing through the inside from the peripheral surface of the rotor 2 is bored to insert a tumbler lock plate 6, and it is opposed to notched teeth 4c of the tumbler 4. When the rotor 2 is rotated without inserting a regular key plate 20 into a key guide hole 11, an interlocking pin 7 is slipped off from a thread 1b, the tip of the plate 6 is interlocked with teeth 7c, and the rotor 2 is uncapable of turning. Accordingly, dishonest unlocking can be prevented.