To prevent an illicit unlocking operation such as picking or the like by fitting a coil spring and a tumbler piece from both side surfaces of a cylindrical cylinder body so as to be mutually opposed.

In this cylinder lock mechanism, fitting grooves 4 for the tumblers 3 having spring insert holes 6 at one-side ends are provided at proper intervals on both side surfaces opposed in the diameter direction of the cylinder body 1 having a keyhole 2 at one end. The tumbler pieces 3 are fitted to the insert holes 6 and fitting grooves 4 together with the coil springs 5, respectively, and the tumbler pieces 3 energized by the springs in the lock body are slid in opposition to each other in the diameter direction.