PURPOSE: To protect a cylinder lock from the violent breakdown by arranging a hindering member inside of a cylinder housing, and constraining a latch immediately in the case where a main part of the cylinder lock is removed from the housing.

CONSTITUTION: A block pin 9 is arranged in a hole, which is extended in a cylinder housing 1 in the axial direction and opened toward a latch 5, freely to be moved, and pressurized by a spring 10 in a direction of the latch 5. A lock pin 12 is arranged so as to constrain the movement of the block pin 9 in the case of the normal lock condition. In the case where a cylinder plug 2 is drawn by a violent means or the like, the lock pin 12 is moved immediately so as to release the constraint of the block pin 9. The block pin 9 is moved forward while being pressurized by the spring 10, and a front end of the block pin 9 progresses in a passage of the latch 5 so as to constrain the latch 5.