1. An image processing apparatus, comprising: a resolution unit resolving an image signal into a plurality of frequency components whose spatial frequencies are different from each other; an extracting unit extracting a variation component of the image signal based on each of the plurality of frequency components, an absolute value of the variation component being smaller than a predetermined reference value; a calculating unit calculating a level difference amount of the image signal based on the extracted variation component; and a subtracting unit subtracting the calculated level difference amount from the image signal.