To prevent thumb turn rotation by making a thumb turn axis irrotational to the unlocking direction.

The thumb turn decice is provided with a thumb turn axis 12 making the dead bolt 3 reciprocate by the rotary motion, a knob 30 detachably engageable to a thumb turn axis 12 to rotate the thumb turn axis 12 by fitting to the thumb turn axis 12. a fitting seat 20 holding not to drop out the knob 30, and a cover 40 put on the seat 20. The thumb turn axis 12 is turnably retained on the seat 20, the top end of the knob 30 is held not to drop out in a recess 21 formed in the seat 20. By magnetizing a magnet 37 of the knob 30 to a magnet 16 of the thumb turn axis 12, the connection state of the knob 30 and the thumb turn axis 12 can be held, In the case that the connection is disconnected, the connection is induced by the attraction of each magnet and their impulsive force.