1. An image processing apparatus comprising: a dividing unit for dividing an input image into a plurality of divided images; a difference image calculation unit for calculating a difference image between a divided image prior to noise removal and a divided image after noise removal with respect to each of the plurality of divided images; a relative value calculation unit for calculating a relative value between a pixel intensity in the divided image prior to noise removal and a pixel intensity in the difference image with respect to each of the plurality of divided images; a distribution detection unit for detecting a frequency distribution of relative values in a background area of the input image, contained in frequency distribution of relative values calculated with respect to the plurality of divided images; and an estimation unit for estimating an intensity of noise in accordance with the frequency distribution of the relative values in the background area.