To provide a low-cost crime-preventing thumb-turn, which prevents lock-picking, is of a simple structure with a small number of component parts, and can be operated securely and steadily.

An outer cylinder 80 is fitted with a tab 2 and a thumb shaft 50 at its rear and forward ends respectively, both rotatably. The shaft of the aforesaid tab is engaged to the thumb shaft so that the rotation of the tab is transferred to a door key L by the tab's integrally turning the thumb shaft. A pair of button members 30 and 40 are fitted to the tab. When the key is locked, buttons 31 and 41 of the button members protrude from the tab, while latch parts 33 and 43 formed on the button members engaging into a latch groove formed on the outer cylinder to regulate the rotation of the tab. The aforesaid rotation regulation is released by pressing the aforesaid buttons in. A rotation-transferring mechanism, in which a control part formed at the forward end of the aforesaid pair of button members is engaged to a control groove formed on the thumb shaft, is arranged so that the rotation regulation on the tab is released by operating the cylinder from the outside of the room via the aforesaid rotation-transferring mechanism.