1. An object detecting method for detecting an object in a predetermined monitor area, comprising the steps of: imaging a plurality of different areas in said predetermined monitor area having no object at a predetermined time by an imaging device and registering respective images corresponding to said different areas having no image of said object to be detected; at a different time from said predetermined time, imaging said predetermined monitor area by said imaging device; comparing an image from said imaging device with a corresponding one of said plurality of registered images; and detecting said object to be detected based on a result of the comparison.