The invention concerns with an exposure control mechanism for use in an image pickup apparatus in which a diaphragm aperture formed by a plurality of diaphragm blades moving straightforward in opposite directions is covered by an ND filter, and intends to prevent deterioration of image quality caused by diffraction even in an image pickup device having a small picture size and a short pixel pitch. The image pickup apparatus comprises an exposure control mechanism for adjusting the quantity of light flux entering a shooting lens system. The exposure control mechanism comprises a diaphragm made up of diaphragm blades movable on a plane perpendicular to an optical axis in opposite directions to define a diaphragm aperture, and an ND filter made up of at least two ND filter elements having different transmittances. When the diaphragm blades are displaced from an aperture open state in a direction to restrict the quantity of transmitting light, an aperture area is restricted by the diaphragm blades from the open state to a predetermined aperture area, and thereafter the ND filter is advanced into the diaphragm aperture successively from one of the ND filter elements having the highest transmittance while the predetermined aperture area is maintained.