In an exposure control apparatus for a vehicle-installed camera which captures images of an external scene ahead of the vehicle as arrays of picture elements, a measured brightness value is obtained from each image based on respective spatial-domain average brightness values of a plurality of picture element groups, such as horizontal rows of picture elements. For each of at least part of the groups, time-axis filtering is applied to a plurality of average brightness values that have been successively obtained for that group, to obtain an average brightness values with reduced effect of time-axis variations in brightness.