To provide a thumb turn structure which allows a security function for restricting the rotation of a knob part to be maintained in a release state without use of a separate member.

A thumb turn structure comprises: a button 39 which projects from the external surface of a knob part 15 and releases the restriction of an internal thumb turn rotation-restricting mechanism when pushed in; and a switch member having a portion exposed to the surface, which is disposed at the knob part 15 so as to be slidable in the orthogonal direction to the push-in direction of the button 39, and engages the pushed-in button 39 at one slide position 67 so as to retain the button in the push-in position. Preferably the thumb turn structure is provided with a pair of buttons 39 to release the restriction of the thumb turn rotation-restricting mechanism when pushed-in together at the knob part 15, and the switch member retains the pair of concurrently pushed-in buttons 39 in the push-in position.