1. An intruding object detecting method comprising the steps of: inputting images of a monitoring visual field from an image pickup device; storing said images from said image pickup device in a memory device; calculating for each pixel a difference in luminance value between an input image from said image pickup device and each of a plurality of different images outputted from said memory device to obtain respective differential images; adding said respective differential images, each of which is given weight with predetermined proportion to generate a synthesized differential image; binarizing said synthesized differential image based on a predetermined threshold value to generate a binarized image; and detecting an object in said binarized image as an object intruding within said monitoring visual field.