According to the pattern search method of the present invention, detection of a detection image region relative to an observation image is performed by a pattern matching. An image whose size is substantially the same as that of the observation image is used as the detection image region. The detection image region is relatively displaced with reference to the observation image, thereby detecting a degree of similarity for a common region which is common to both of the region and the image. Displacement range of the detection image region is set in advance. If area of the common region is larger than a predetermined value, calculating the degree of similarity will be performed.