A method of updating a reference background image used for detecting objects entering an image pickup view field based on the binary image generated from the difference between an input image and the reference background image of the input image. The image pickup view field is divided into a plurality of view field areas, and a reference background image corresponding to each of the fixed divided view field areas is updated. An entering object detection apparatus using this method has an input image processing unit including an image memory storing an input image from an image input unit, a program memory storing the program for activating the entering object detecting unit, a work memory and a central processing unit activating the entering object detecting unit in accordance with the program. The processing unit has an entering object detecting unit determining intensity difference for each pixel between the input image and the reference background image not including an entering object to be detected and detecting an area with the difference larger than a predetermined threshold as an entering object, a dividing unit dividing the image pickup view field of the image input unit into a plurality of view field areas, an image change detecting unit detecting the change of the image in each of the divided view field areas, and a reference background image updating unit updating each portion of the reference background image corresponding to each of the divided view field areas associated with the portion of the input image free of the image change, wherein the entering object detecting unit detects entering objects based on the updated reference background image.