1. A detecting method of detecting a specific object region located in a picture frame recorded by an image sensor, comprising steps of: creating picture frames by picking up an object sequentially; acquiring a shift amount between a present picture frame presently created among said picture frames and a previous picture frame created prior to said present picture frame among said picture frames; shifting an outline frame defined around said previous picture frame according to said shift amount on said previous picture frame, to form a normalized picture frame from an image region surrounded in said outline frame after shifting; retrieving a difference between said present picture frame and said normalized picture frame, to detect a difference region being according to said difference and included in said present picture frame; detecting said specific object region in said difference region.