To eliminate the adverse effect of a click mechanical section if required for a purpose, such as the recording (sound collecting) quality improvement of a video camera function and avoid troubles such as operation mistakes due to the difference in operating force of a diaphragm ring.

A device for a lens diaphragm comprises a click mechanism section Uc and a mode switching mechanical section Um. On one of an inner peripheral surface 3i of a diaphragm ring 3 or an outer peripheral surface 2cf of a fixed barrel section 2c is disposed an engaging section 5 having an engaging body 4 supported elastically. On the other of them is disposed an engaged base section 6 having an engaged surface 6f that forms a plurality of engaged sections 6c... with which the engaging body 4 pressure-contacts and with which the engaging body 4 can engage at positions corresponding to diaphragm values when the diaphragm ring 3 is rotated. By displacing either the diaphragm ring 3 or the engaged base section 6 in an axial direction Fs, the mode switching mechanical section Um can switch between a click mode Mc in which the engaging body 4 and the engaged surface 6f pressure-contact each other and a non-click mode Mn in which the engaging body 4 and the engaged surface 6f separate from each other.