To provide a thumb-turn device which can surely prevent a fraudulent operation from the outside of the room by making a one-handed operation difficult.

This thumb-turn device comprises: an operation shaft 3 which is connected to a dead bolt operating part 2 of a lock device 1; an operation knob 4 which can be freely operated for a shift between a slipping position and a connection position to be connected to the operation shaft 2 in a turning direction, and which is urged to the side of the slipping position; and an operation regulating member 5 which can be freely operated for a shift between a position to regulate the provision of an operating physical force in the direction of connection between the operation knob 4 and the operation shaft 3, and a position to permit it.