An automatic exposure control system permitting for a photographic camera to make an exposure in either of the shutter preselection automatic exposure range and the diaphragm preselection automatic exposure range is established by combination of an adapter unit with the camera. This adapter unit has a housing attachable to and detachable from the camera body, and includes an exposure operation and control circuit capable, upon preselection of an exposure time, of deriving an exposure value representing an effective exposure aperture, and, upon preselection of diaphragm value, of deriving an exposure value representing an exposure time. Responsive to the output of the adapter unit, the control device in the camera body controls operation of the shutter and diaphragm mechanisms thereof selectively in accordance with the exposure value derived from said exposure operation and control circuit. The provision for permitting the camera to make an exposure in an automatic flash exposure range is also made at the exposure operation and control circuit of the adapter unit in a manner to determine the period of energization of a flash unit associated with the camera.