The present invention relates to an image processing device and a method for processing an image. The image processing device includes a brightness improvement unit to improve a brightness of inputted images by increasing a brightness of a dark image more darker than a knee point among inputted images with reference to a predetermined knee point and decreasing a brightness of an image more brighter than the knee point; and a contrast improvement unit to improve a contrast of the image outputted from the brightness improvement unit. The image processing device and the method for processing the image can reduce a distortion phenomenon and a saturation phenomenon or the like by reflecting the characteristics of images during the brightness improvement and contrast improvement of the images.