In order to examine a degree of monocular diplopia is easily examined in a quantitative manner, a plurality of circular eye marks are seen from a predetermined examination distance with a single eye of a patient to find a particular eye mark whose double vision images are observed to be circumscribed each other, and a degree of monocular diplopia is determined by a view angle of the relevant circular eye mark. Since the eye mark is formed as a circular pattern, a degree of monocular diplopia can be examined irrespective of an axial direction along which double vision images appear.
