To provide a diaphragm aperture correction mechanism able to make a highly accurate correction despite its simple configuration and free from hazardous light such as ghost, and to provide a lens barrel.

A diaphragm aperture correction mechanism comprises: a diaphragm unit having a plurality of diaphragm blades, a base plate (aperture) supporting the diaphragm blades such that these blades are freely rotated, and a rotating member (diaphragm) which rotates on an optical axis to move the plurality of diaphragm blades in an opening/closing direction; a movable member (third-group lens chamber) rectilinearly movable in an optical axis direction as a focusing operation is performed; and a correction cylinder having a cam groove engaged with a roller fixed to the movable member (third-group lens chamber). The correction cylinder has a guide member engaged with a key portion provided on the base plate (aperture). The movable member (third-group lens chamber) rectilinearly moves, and the correction cylinder thereby rotates around the optical axis on the basis of the relation between the roller and cam groove.