1. An apparatus for detecting a moving object in dynamic images, comprising: image input means for sequentially taking input images of the moving object against a static background; difference calculation means for obtaining difference images from successive input images taken by said image input means as picture elements; noise model estimation means for estimating an appropriate noise model representing a lightness variation due to an image taking environmental condition under which the input images are taken by the image input means, the estimation being made according to the input images taken by the image input means and the difference images obtained by the difference calculation means by estimating values of noise model parameters in a regression model, and specifying a ratio of a difference in lightness at each picture element of each difference image with respect to an average difference in lightness over an entire imaging view field of each difference image as a function of lightness at each picture element in each input image expressed in terms of the noise model parameters; change determining means for determining changed regions in the entire imaging view field by compensating for the lightness variation by subtracting from the difference images obtained by the difference calculation means an estimated lightness variation based on the noise model estimated by the noise model estimation means; and detecting means for detecting the moving object in the dynamic images according to the changed regions determined by the change determining means.