To achieve speeding-up of a diaphragm aperture opening/closing operation without causing upsizing of a device and an increase in power consumption.

A bearing mechanism 120 includes: a rear lens barrel 101a including a holding part that holds a plurality of steel balls 302 rotationally movably along the periphery thereof; a ball pressing ring 303 that controls the movement of the steel balls 302 so that the steel balls 302 may rotationally move along a steel ball holding surface 501; and a preset ring member 301 that comes into contact with the steel balls 302 and rotates with the rotational movement of the steel balls 302. A surface treatment by " Mitami Lyte (R)" is applied on the surfaces of the ball pressing ring 303 and the preset ring member 301 composed of an aluminum material.