1. An object detecting method for detecting an object in comparison between an input image from an image pick-up device having a zoom mechanism and a template image stored comprising the steps of: storing a first image within a view field of said image pick-up device as said template image; recording a power of said zoom mechanism to be changed; picking-up a second image to be detected from said image pick-up device; changing a size of either one of said template image and said second image on the bases of said changed power of said zoom mechanism; and performing a template matching between said template image and said second image to detect said object.