To provide a cylinder lock which facilitates insertion of a duplicate key (key) into a keyhole thereof, and has an excellent appearance, and to provide a plug for use therein.

According to the cylinder lock, a countersink-shaped recess 15 is formed in a front end surface of a head of the plug 10, and the recess 15 has a taper surface 16 expanding frontward from a rear portion thereof. Then, the keyhole 12 facing to the countersink-shaped recess 15 of the plug 10 has a depression 20 at least at one end of the keyhole 12 in a width direction, for engaging with an engaging portion K2 formed on a main body K1 of the key K, and guiding the key K into the keyhole 12 upon abutment of a front end K3 of the key K on the depression 20. Further, the depression 20, the whole of the keyhole in terms of a thickness direction 12b, and at least one end of the keyhole in terms of a width direction 12a are enclosed by the countersinkshaped recess 15.