1. An exposure control apparatus for exposure control of an electronic digital camera installed on a vehicle, said camera disposed to capture an external scene as an image comprising an array of picture elements having respective luminance values, said image expressing a target object located upon a surface of a road extending ahead of said vehicle, said exposure control apparatus comprising circuitry configured to convert said luminance values to corresponding brightness values of said external scene, based upon a predetermined relationship between an exposure condition of said camera, said luminance values, and said brightness values; wherein said exposure control apparatus comprises extraction circuitry configured to extract from said image a first plurality of picture elements to constitute a target object-use region for use in measuring a brightness value of said target object and a second plurality of picture elements to constitute a road surface-use region for use in measuring a brightness value of said road surface, and exposure control circuitry configured to control said camera exposure, based upon said brightness values of said road surface-use region of said image and said brightness values of said target object-use region of said image; and wherein said road surface-use region and said target object-use region are disposed at respective predetermined fixed locations within said image, said road surface-use region and said target object-use region are formed of respective fixed numbers of lines of picture elements, and respective numbers of picture elements constituting said lines of said target object-use region successively increase from an uppermost part of said target object-use region to a lowermost part of said target object-use region.