To improve safety against the illicit unlocking of a lever tumbler lock or a rotary disk tumbler lock while maintaining the strength of a partition plate.

In this cylinder lock against a detection tool, key guides rotatably attached to the front side of an outer tube, tail piece members rotatably attached to the rear side of the outer tube, and partition plates which are biased, by a spring member, to the predetermined space between the tail piece and the key guide in a predetermined direction and have respective movable obstructing elements having portions to be detected for unlocking and respective key holes at the center are arranged parallel to each other in the longitudinal direction of the outer tube so as to face each other. A small projection for obstructing detection for receiving the sensor of the detection tool when the sensor is rotated in the direction of the detection portion for unlocking thereof is provided to at least one side surface bottom end of the partition plate.