1. A method of extracting a circular region from a fringe image by capturing onto a predetermined coordinate system a fringe image in an area to be observed of a sample having a circular region to be analyzed within said area to be observed, and extracting a circular image region representing said region to be analyzed on said coordinate system; said method comprising the steps of: (i) carrying out a binary coding procedure, said binary coding procedure determining fringe intensity information of each pixel on said coordinate system by a predetermined fringe analysis method, determining according to thus determined fringe intensity information a threshold for binary-coding said fringe image in said area to be observed, and discriminating a candidate region having a fringe intensity level of said circular image region in said fringe image and a noncandidate region having a fringe intensity level of a region outside said circular image region from each other by said binary coding according to thus set threshold; (ii) placing, on said coordinate system subjected to said binary coding procedure, a plurality of linear gratings in parallel with each other with a predetermined interval P shorter than said diameter D of said circular image region; (iii) measuring the length of a segment constituted by a part of said linear gratings included within said candidate region; (iv) determining whether or not the length of said segment falls within a predetermined suitable length range of said diameter D or shorter; and (v) carrying out a circular image region selecting procedure, said circular image region selecting procedure placing on said coordinate system a perpendicular bisector of said segment determined to fall within said range, measuring the length of a diameter candidate segment constituted by a part of said perpendicular bisector included within said candidate region, determining whether or not the length of said diameter candidate segment substantially equals said diameter D, and taking an image region including thus positively determined diameter candidate segment as a chosen candidate region of said circular image region, or carrying out said circular image region selecting procedure a plurality of times while changing an inclination of said linear gratings; thereby defining and extracting said circular image region.