1. A pattern search method, comprising: an observation-image input step of inputting an observation image, a detection-image-region input step of inputting a detection image region including image of a detection target, a superimposing step of superimposing said detection image region on said observation image by relatively displacing said detection image region with reference to said observation image in sequence, a common-region detection step of detecting a common region common to both of said detection image region and said observation image which are superimposed at said superimposing step, a degree-of-similarity detection step of detecting a degree of similarity between said observation image and said detection image region in said common region, and a position detection step of detecting a relative position between said observation image and said detection image region at the time when said degree of similarity detected at said degree-of-similarity detection step is the highest, wherein, said common region detected at said common-region detection step is used as said detection image region.