PURPOSE: To easily pull out and replace an inner cylinder by providing a stopper to the rear end of an outer cylinder and by enabling the engagement of the outer cylinder and the inner cylinder to be released against the spring force, with a key for replacing of the inner cylinder used in an unlocked state.

CONSTITUTION: An outer cylinder 2 and an inner cylinder 4 that is incorporated in the outer cylinder are provided and a key slot 4c is made in the inner cylinder 4 in its axial direction. An ascending/descending body 14 is so fitted to the rear end of the inner cylinder 4 that the lower end of the body 14 projects in the key slot 4c. A stopper 17 is so fitted to the rear end of the outer cylinder 2 that the lower end of the stopper is forcibly engaged with the outer edge of the outer cylinder 2 in a locked state. When a key 12 for replacing of the inner cylinder is inserted in an unlocked state, the ascending/descending body 14 that rises being pressed with the tip of the key pushes up the stopper 17, and the engagement to the inner cylinder 4 is released against the spring force of a spring 19. Thereby pulling out and replacement of the inner cylinder 4 by using the key 12 for replacing of the inner cylinder can be made easily.