PURPOSE: To simplify structure and permit the easy replacement with a new kind of cylinder lock by permitting only a rotor having each engagement element to be inserted and pulled out from outside, when the engagement member for a rotor is positioned in a cut groove.

CONSTITUTION: A rotor 20 which can be inserted into the case 10 of a cylinder lock A is installed, and a cut groove 14 is formed over the whole length in the longitudinal direction of the inner peripheral wall of the case 10. Further, at the rear part position of the rotor 20, an engagement member 30 which is not engaged with a key K is installed. The engagement member 30 is positioned in the cut groove 14 (position of Z line), turning the engagement elements 22 and 22 at the positions other than the locking/lock releasing positions by the key K. At this time, keeping each enagagement element 22 with the key K, only the rotor 20 having the engagement element 22 can be inserted and pulled out from outside, and the change to a new kind of cylinder lock is enabled.