PURPOSE: To prevent cut destruction by a drill blade by providing steel balls to be arranged in the front surface of a tumbler pin, in the case of a cylinder lock having the tumbler pin fitted and inserted while being urged by springs to pin holes formed penetrating through inner and outer cylinders.

CONSTITUTION: A lock is provided with an inner cylinder 11 forming a key groove 10, outer cylinder 12 turnably fitting and inserting the inner cylinder 11, operating lever 13 secured to the inner cylinder 11 in its internal end part, tumbler pin 15 pressed by springs 16 in pin holes 14 drilled penetrating through from the inner cylinder 11 to the outer cylinder 12 and steel balls 19 or the like arranged in chambers 17, 18 formed in a front surface of the pin hole 14 fitting the tumbler pin 15 to be inserted. Here the chamber 17 is formed in the front surface from the pin hole 14 of the inner cylinder, fitting with a play the steel ball 19. While the outer cylinder 12 forms in its front surface before the pin hole 14 the chamber 18 fitting with a play the steel ball 19. And the steel balls 19 are freely rotated in the respective chambers 17, 18, preventing the lock from being cut.