To provide a thumb turn device of a locking and unlocking mechanism capable of surely preventing an irregular unlocking by operating a thumb turn from the outside of a door while preventing the loss of the thumb turn as a constitution for always mounting the thumb turn.

The thumb turn device is so constituted that a clutch 32 is retreated and held to make non-conjunction with the thumb turn 25 and a shaft 30 as long as a dial knob 28 is not made to coincide with a graduation, a state of prevention of crimes is made so as to race the thumb turn 25, the movement of a lock bar 33 is allowed in accordance to the coincidence with the graduation of the dial knob 28, the clutch 32 is connected to the thumb turn 25 with clutch connection by making tensile operation of the dial knob 28 under this condition and that the rotation of the thumb turn 25 is transferred to the shaft 30 through the clutch 32 to make locking and unlocking.