PURPOSE: To assemble two locking mechanisms in a cylinder having one key hole and lock or unlock stepwise respective locking mechanisms by two keys different from each other.

CONSTITUTION: A pair of fixing grooves 2a, 2a, a pair of small diameter parts 1a, 1a, a pair of relief grooves 2b, 2b and a pair of large diameter parts 1b, 1b symmetrically formed against the center axial line along the generating line, are connected at different angular positions from one another, at the inside of external cylinder 1 of a disk tumbler lock. And fixing protrusive lines 3 are formed respectively in this large diameter parts 1b. When the first key is inserted in the key hole of the inner cylinder with an outer diameter in which the inner cylinder can rotate without hindrance against the fixing protrusive line, the front ends of respective tumblers are lined up at the shear line along the inside of the large diameter part 1b of the external cylinder and hence, when the second key is inserted, respective tumblers can pass the notched line protrusively formed.