A cylinder lock including a rotor casing formed with a groove and an elongated through-opening, a rotor rotatably received in the rotor casing, tumblers which are respectively formed with projecting ends engageable with the groove and slidably fitted into the rotor, a tumbler blocking member slidably inserted into the elongated through-opening, a plunger and a plunger driving device. By the above arrangement, the cylinder lock can be positively unlocked either by using a key or without using the key and instead using the plunger driving device and driving the tumbler blocking member.