Provided are an apparatus and method for automatically adjusting luminance, and more particularly, apparatus and method for automatically adjusting luminance per region in plurality of luminance regions of an image to be displayed according to its average luminance value. An average luminance of an image is detected, and then contrast and brightness of image is automatically adjusted according to detected average luminance. Amplification of contrast and brightness is increased to brighten an image in an image with low luminance and reduced to suppress image dazzle in an image with high luminance, thereby providing user with an image of appropriate brightness, irrespective of original brightness. It is possible to prevent diversion of image by dividing its luminance level into a plurality of regions and controlling contrast and luminance per region. Further, a buffer region between luminance regions prevents flickering of image on a screen due to diversion of an adjustable gain.