1. A monitoring device which monitors a region to be monitored, said device comprising: an image-capturing unit operable to capture an image within the monitored region, and to generate an overall image; a moving object image generation unit operable to generate, from the overall 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 operable 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.