To provide a cylinder lock having high resistance to picking.

This cylinder lock allows and prohibits rotary operation to a rotor 2 by moving a tumbler 1 to a prescribed position in the rotor 2 by an unlocking code forming recessed part 6 arranged on a surface of a blade part 5 of an inserted key plate 4 by inserting and forming the rotor 2 having the tumbler 1 in a case 3. The rotor 2 is provided with a foreign matter detecting tumbler 7 freely movable in the moving direction of the tumbler 1, and having one end positioned on a rotary boundary surface to the case 3 of the rotor 2 when the key plate 4 is not inserted. The foreign matter detecting tumbler 7 is provided with a detecting projection 9 fitted to an engaging groove 8 recessed on a side wall surface of the key plate 4, and holding the foreign matter detecting tumbler 7 in a position when the key plate 4 is not inserted even in a state of inserting the key plate 4. The foreign matter detecting tumbler 7 is arranged so that the detecting projection 9 is energized inside the rotor 2.