To provide a cylinder lock which enables the setting of an unlocking code to be more easily changed than ever.

When a release key member 2 is inserted into a keyhole 21 of a cylinder 20, a displacement portion 42 pushed by a pushing protrusion 210 is displaced, and the fitting of a biasing protrusion 44 and a fitting groove 32 is released so that the cylinder 20 disconnected from a transmitting member 30 can be turned to a predetermined rotational position. In this state, a part of a side bar 29 is expanded by being fitted into a storage groove 16 which is formed on the inner periphery of a case 10. This makes a combination of tumbler pieces 27 changeable. When another ordinary key member 3 is inserted into the keyhole 21, the combination of the tumbler pieces 27 is respectively changed according to the unlocking code. Subsequently, when the cylinder 20 is returned to the original rotational position with the ordinary key member 3, the combination of the tumbler pieces 27 is fixed, and the transmitting member 30 and the cylinder 20 are reconnected to each other. Thus, the setting of the unlocking code of the cylinder 20 is changed.