To minimize the number of parts such as input members and operation obstructing elements on a knob side.

This crime prevention thumb-turn comprises a thumb-turn shaft 6 rotatably installed in a fixed seat and having a locking groove 10 in the outer peripheral surface on the outer end side, a cylindrical knob member 15 so fitted onto the thumb-turn shaft as to enclose the locking groove, a resilient member 21 installed in the knob member and having a support hole 23 opposed to the locking groove, and an engagement body 25 inserted through the support hole of the resilient member and having an inner end engageable with and disengageable from the locking groove and an outer end projecting from the insertion hole 20 of the knob member. When the outer end 25b of the engagement body is pressed in, the engagement body is displaced against the resilience of the resilient member, and the inner end 25a is engaged with the locking groove. Consequently, the thumb-turn is changed over from a crime prevention mode in a slip state to a normal mode in which the knob member and the thumb-turn shaft are rotated together.