To provide an unlocking prevention device and a fixture therefor which reduces a load upon unlawful unlocking, and can be adapted to the difference in the type of a cylinder lock or in the shape depending on manufacturers.

A rod-like stopper is disposed to be extendable from a box body having a plurality of fixture means to a door, and at a position close to an unlocking thumb turn mounted on the door. The unlocking prevention device and the fixture used for it function by driving the stopper by a drive mechanism. By extending the stopper to be positioned at the unlocking thumb turn, the thumb turn is prevented from rotating, and by returning the stopper to its original position, the door can be unlocked.