A lens barrel includes: a barrel body; a diaphragm pivotally held in the barrel body; a diaphragm operating member that changes the diameter of the diaphragm; an operation arm that can pivot in the barrel body and fits in the diaphragm operating member, the operation arm, when pivoted, changing the diameter of the diaphragm via the diaphragm operating member; and a light-blocking plate that prevents unwanted light from entering the barrel body and has a passage portion through which the operation arm passes and an engagement portion that an engaging piece protruding from the diaphragm operating member engages, the light-blocking plate, along with the diaphragm operating member, pivotally supported, wherein the passage portion has an opening having a width that is substantially the same as the width of the operation arm.