To make it hard to be illegally unlocked by improving a groping preventive function on a cylinder lock furnished with a plate tumbler.

An engagement part 7 with a key 12 attached with a height difference in accordance with a gage is provided to one side of an insertion hole 6 of the key furnished at a central part of a plate tumbler 5, and a coil spring 8 to energize in the locking direction from below the central part of the tumbler is made contact with it.