1. A monitoring device which monitors a region to be monitored, said device comprising: an image-capturing unit to capture an image within the monitored region, and to generate an input image; a moving object image generation unit to generate, from the input image generated by said image-capturing unit, a moving object image that is an image of moving objects or of a crowd; and a density calculation unit to calculate, based on the moving object image generated by said moving object image generation unit, a degree of density of the moving objects or of the crowd, as a density that is parameterized by a function approximation or a frequency transformation, wherein said moving object image generation unit generates the moving object image by performing differential processing on the input image, wherein said density calculation unit transforms the moving object image on which the differential processing has been performed, into one-dimensional information, and calculates the density based on the transformed one-dimensional information, and wherein said density calculation unit transforms the moving object image on which the differential processing has been performed, into one-dimensional information, by: binarizing each of a plurality of pixel values of the moving object image on which the differential processing has been performed, through conversion into either a first value or a second value; scanning the binarized moving object image in a direction of a vertical axis from a top part of the moving object image for each of a plurality of values on a horizontal axis of the moving object image; and projecting, to the horizontal axis, detected vertical axis values each of which is a maximum value on the vertical axis of a pixel which has been detected first in the scanning and has the first value.