In order to inspect quality of the silicon wafer surface region where devices are to be fabricated, an instrument named Optical Shallow Defect Analyzer (OSDA) was developed. The OSDA can estimate the size and depth of the defect by measuring scattered light from each defect at two wavelengths having different penetration depths in Si. The principle of OSDA measurement is based on the Beer-Lambert law. In this review, the validity, the limitation and applications of the OSDA measurement are described. The validity were checked by TEM observation of the defects detected with OSDA. The applications include measurements of grown-in defects in various Czochralski (CZ) Si wafers and epitaxial wafer, and discrimination between inner defects and surface pits, e.g. crystal originated particles (COPs) induced by thermal oxidation.