To provide a thumb-turn turning preventive implement capable of positively preventing turning of a thumb-turn.

The preventive implement for preventing the illegal turning of the thumb-turn 3 projected from the surface of a door 2 to open/close a key of the door 2, using an illegal instrument 20 to perform illegal operation of the thumb-turn 3 from the outside, is provided with a preventive implement body 4 disposed at the outer periphery of the thumb-turn 3 of the door 2 and formed of a heat resistant material capable of resisting the heat of the heated illegal instrument 20. The preventive implement body 4 is preferably formed in cylindrical body shape, particularly preferably formed in cylindrical body shape with the inner surface gradually enlarged in diameter from one end on the surface side of the door 2 to the other end.