1. A pattern search method, for searching a pattern with a template, using a pattern search device, the method comprising the following steps implemented by the pattern search device: inputting an observation image with an amount of width which is substantially similar to width of an image of the template, relatively displacing the image of the template with reference to said observation image in sequence, with changing a width of a common region between the image of the template and said observation image defined as an overlapped area of the image of the template and the observation image, shapes of the observation image and the image of the template in the common region being equal with each other, judging whether a predetermined value regarding the width of the common region is equal to or greater than a threshold value, and relatively calculating degree of similarity between the observation image and the template for the pattern search, in accordance with an amount of the width of the common region.