To provide a picking prevention device of a cylinder lock capable of preventing the cylinder lock from being unlawfully unlocked because the new developed cylinder lock is so constituted that a usual picking tool can not be used.

The conventional cylinder is so constituted that four tumblers 7 are used in an inner cylinder, a special tumbler 4 and a special tumbler push spring 5 are newly provided to the middle parts of (the second and the third of the tumblers) of them, and an action (foreign matter intrusion controlling action) different from the conventional tumblers 7 is carried out. For example, when the conventional type key or picking tool is inserted from a key hole, it comes into contact with the special tumblers at the middle parts, and when it is forced to push in the inner part by pushing the special tumblers aside, the special tumblers are so constituted that they are in a locking state, and picking operation can not be done while the foreign matter is in the key hole.