PURPOSE: To detect the depth including an abnormal part by transmitting a coherent optical beam into an optically isotropic substance, dividing the transmitted light into two components by a beam splitter and detecting the intensity of light by polarizers and detectors.

CONSTITUTION: Coherent light having a beam waist is provided with an electric vector E, a magnetic vector H, a painting vector S. The light is made incident upon the isotropic substance 1 and the transmitted light is projected. The projected light is divided into two light components 5, 6 by the beam splitter 4. Respective light components 5, 6 are passed through respective polarizers 7, 8 and electric signals en, ea indicating the intensity of light are obtained by the photodetectors 9, 10. The substance 1 is moved, and if a defect 2 exists in an optical path, the electric signals en, ea are generated differently. Since the transmitted light is divided into two components by the beam splitter 4 and detected, the abnormality in the isotropic substance can be detected and the abnormal part can be detected in the thickness direction of the substance.