To prevent unlocking malpractice with an inserted wire or rod hitched on a thumb-turn to be rotated.

This anticrime thumb-turn unit C to be mounted on a door with a key box comprises a thumb-turn box 11 to be mounted on the indoor side of the door, an operating member 19 having a key-shaped insertion portion 19b, a key guide 21 turnably provided in the thumb-turn box 11 and having a key hole 21a into which the insertion portion 19b of the operating member 19 is inserted and which is exhibited from an operation hole 13 in the thumb-turn box 11 to the indoor side, a thumb-turn core 25 turnably provided in the thumb-turn box 11 for following the turn of the key guide 21 when connecting its front end portion 25a to the key box A to input operating force, and an opening/closing plate 29 slidably provided on the thumb-turn box 11 for opening the operation hole 13 at one sliding position and closing the operation hole 13 at the other sliding position.