The present invention relates to a light quantity adjusting device, a lens barrel and an image pickup apparatus. The light quantity adjusting device includes a base formed with a window; first and second diaphragm blades provided on the base and adapted to adjust a size of a diaphragm aperture; first and second cam-pins adapted to shift the first and second diaphragm blades; and a plurality of guide pins provided on the base so as to project therefrom.