1. An image contrast enhancement apparatus, used to process an original image, the image contrast enhancement apparatus comprising: an image analysis module, statistically analyzing pixels of each of M computation windows to produce a plurality of histogram bins, and combining the histogram bins corresponding to each computation window into a sub-histogram, wherein M is a positive integer, sizes of the computation windows are not identical, and each of the histogram bins presents a total pixel number corresponding to a pixel value; and a local contrast enhancement module, enhancing at least a local contrast for each of the computation windows, and generating M enhanced images, wherein the local contrast enhancement module comprises: a histogram separation unit, sequentially separating the histogram bins in each of the computation windows into L separation levels according to a tree structure, separating the sub-histogram in each separation level into two sub-histograms in the next separation level, and increasing or decreasing a base width of each sub-histogram, such that group densities of the two sub-histograms are identical, wherein L is a positive number and a separation level number; a histogram adjustment unit, re-distributing the histogram bins in each sub-histogram, or merging a portion of histogram bins in each sub-histogram; and a local contrast enhancement unit, enhancing the at least one local contrast of the original image according to the computation windows, and generating the enhanced images.