To provide a pin cylinder lock hard to be picked and having high crime prevention property.

This pin cylinder lock is formed so that an inner cylinder 5 having a pin hole and a key hole 8 is rotatably fitted to an outer cylinder 1 having a pin hole, an upper pin inserted into the pin hole of the outer cylinder 1 is energized to the inner cylinder 5 side by a spring 3, lower pins 7a to 7f are slidably fitted to the pin hole of the inner cylinder 5, and when a boundary line of both upper and lower pins is aligned with a share line L which is the boundary line of the outer cylinder 1 and the inner cylinder 5 by a spare key K, the inner cylinder 5 can be rotated. One or a plurality of communication holes 10 communicating with the pin hole of the inner cylinder 5 are formed in the inner cylinder 5, and a locking member 20 is installed in the communication holes 10 normally in the energized state in the outer cylinder 5 direction by a spring 26. Also, an engagement groove 30 stopping the rotation of the inner cylinder 5 by locking the locking member when the lock is illegally unlocked with an illegal unlocking tool is formed in the outer periphery of the lower pin crossing the locking member 20.