PURPOSE: To prevent the slip-out of a knob while facilitating the removal of the knob.

CONSTITUTION: In the attachment structure of the knob 31 operating an encoder 1, a pawl-shape projection 33 is provided to the outer periphery of the fitting part 32 of the knob 31 with the operation shaft 2 of the encoder 1. The projection 33 comes into contact with the through-hole in which the fitting part 32 is inserted of the panel 25 to which the encoder 1 is fixed by a screw 30 in the vicinity of the peripheral edge of said hole to prevent the detachment of the knob 31 from the encoder 1. The hinge part 28 integrated with the panel 25 and the groove part 29 continuing to the through-hole are provided in the vicinity of the peripheral edge of the through-hole of the panel 25. The hinge part 28 is provided at the position coinciding with the position of the projection 33 when the operation shaft 2 returns to a central position in such a state that the knob 31 is attached to the encoder 1. The groove part 29 is provided at the position coinciding with the position of the projection 33 when the operation shaft 2 of the encoder 1 is fully revolved in a counterclockwise direction to stop.