1. An abnormal shadow detecting system comprising a mammary gland distribution map generating means which generates a mammary gland distribution map of each of left and right breasts by dividing an image of each of the left and right breasts into a plurality of ranges according to the density of the image on the basis of image data representing the image, and an extracting means which extracts a bilaterally asymmetric range by comparing the ranges of the mammary gland distribution map for one of the left and right breasts with those of the mammary gland distribution map for the other breast.