To provide a diaphragm device that has a small outer diameter, and is easy to be attached to a lens barrel.

When a rotation member 130 is rotated to a base 120, a diaphragm device 100 is moved to a position where a diaphragm blade covering an attachment screw hole 126 exposes the attachment screw hole 126, and the attachment screw hole 126 and a screw insertion hole 133 are overlapped with each other. At this moment, a whole of the attachment screw hole 126 is exposed. Under this state, an attachment screw is inserted from the screw insertion hole 133 and is screwed with the attachment screw hole 126, and thereby the diaphragm device 100 can be attached to a lens barrel.