A diaphragm device is provided and includes: a diaphragm mechanism having a diaphragm vane and a diaphragm frame receiving the diaphragm vane; a diaphragm-mechanism cover fixed to a lens barrel to which the diaphragm mechanism is attached, the diaphragm-mechanism cover clamping the diaphragm mechanism with a receiving surface of the lens barrel. The diaphragm mechanism and the diaphragm-mechanism cover are inserted in the lens barrel through a side surface of the lens barrel, and a part of the diaphragm mechanism and a part of the diaphragm-mechanism cover protruding outward relative to the side surface of the lens barrel. The diaphragm device further includes a supporting member in a portion of the diaphragm mechanism protruding outward relative to the side surface of the lens barrel, the supporting member supporting the diaphragm mechanism in a direction of the diaphragm mechanism detachable from the lens barrel.