A measuring method capable of automatically analyzing quantitatively the inner state of a disk is provided. Linearly polarized light from a light source enters a retarder to produce a desired elliptically polarized state. The elliptically polarized light is then passed through a half-wave plate to rotate the direction of a principal axis of the ellipse. The light is expanded into two dimensions by lens systems and to obtain planar information, and is further transmitted through a disk substrate so that the birefringence of a specimen, which depends on an inner stress state and a polymer orientation state, changes the phase of the light. The light wave with its phase changed is passed through a polarizer arranged perpendicular to the principal axis of the retarder. The CCD detects the light wave as a light intensity.