An automatic exposure control apparatus employed by an image sensor module including a lens unit, an image sensing unit having a shutter, and an image signal processing unit, the apparatus including: a brightness calculator extracting average brightness of a image signal; a brightness adjustment determiner determining whether to fix or adjust a speed of the shutter based on the average brightness; an adjustment path controller controlling the speed of the shutter by using the brightness error; a digital filter filtering the brightness error; a shutter speed adjuster adjusting a present shutter speed value by using one of the brightness error and the filtered brightness; and a shutter speed controller controlling a speed of the shutter by using the adjusted shutter speed value.