To provide a thumb-turn device which exerts high durability against a fraudulent operation from the outdoor side, and which does not decrease operability from the indoor side.

This thumb-turn device comprises: a thumb-turn shaft 2 which is formed at the leading end of a shaft body 1, so that a lock device can be operated for locking/unlocking by a turning operation for an operating leading end exposed on the indoor side; an idle-turning cover 4 which is externally fitted to the thumb-turn shaft 2 in an idly turnable manner so as to cover the operating leading end, and in which an exposed window 3 for permitting the application of a turning-operation force to a turning leading end by exposing the operating leading end is provided in an opened manner on a peripheral wall; and a clutch means 5 which is interposed between the thumb-turn shaft 2 and the idle-turning cover 4, and which connects the thumb-turn shaft 2 and the idle-turning cover 4 together in a turning direction when the thumb-turn shaft 2 is turned by a predetermined angle.