To provide a thumb-turn device which is highly safe.

The thumb-turn device includes a thumb-turn case 2 rotatably holding a thumb-turn shaft 1 connected to a lock device, and a knob member 4 removably inserted into a mounting opening 3 of the thumb-turn case 2 and capable of rotating the thumb-turn shaft 1 between locking and unlocking rotational positions in the inserted state. The thumb-turn case 2 is provided with a stopper 6 restricting removal of the knob member 4 by blocking a route of movement of a fall-off preventive part 5, which is arranged on the knob member 4, in a direction of removal when the knob member 4 is in a locking rotational position, and allowing the knob member 4 to be removed by opening the route of movement when the knob member 4 is in the unlocking rotational position. The fall-off preventive part 5 of the knob member 4 and the stopper 6 are formed so that they may be elastically engageable with each other by operation of the knob member 4 in the direction of insertion.

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