PURPOSE: To exchange a cylinder lock only by a cylinder lock used by a user, to prevent malfeasant unlocking (picking) and to form structure, in which an exchanging function is simplified, even when a key inserting port for a rotor, into which a key is inserted, is positioned at the center of the rotor.

CONSTITUTION: A key inserting port 12 in a rotor 10 is formed at the center of a rotor 10, and a rotary member 30 is engaged with an annular recessed groove 14 formed on the front end 11 side of the rotor 10. The rotary member 30 is positioned in the rotational locus of an auxiliary member 20 fitted on the front end 15 side of a key inserting groove 13. When a key K is moved forward along the key inserting groove 13, the front end K' of the key K is engaged with the auxiliary member 20, the auxiliary member 20 releases the engagement of the rotor 10 and the rotary member 30 when the front end K' is abutted against the rotary member 30 with the revolution of the key K and pushed against a spring 50, and the rotor 10 can be extracted together with the key K.