To provide a measuring method and its device capable of measuring simultaneously accuracy of a mounting position on the object surface, when measuring flatness of the object surface formed by an imaging element such as an imaging element surface of a digital camera.

A chart pattern image 2 is emitted from a reference point to a plurality of points on the object surface Oj comprising the imaging element, to thereby form images on a plurality of measuring points set on the object surface, and a reflected image from each point is detected optically-electrically individually at a conjugate point of the reference point respectively, and the flatness of the object surface is measured based on detected values. At that time, the second measuring point used as a reference not interfering with the measuring points is set on the object surface Oj, and the second measuring point is irradiated with spot light, and the accuracy of the mounting position on the object surface Oj is measured based on position data acquired from the imaging element on the object surface Oj by irradiation light.