An optical apparatus for defect inspection having an illuminating optical system for irradiating illumination light beams on the surface of a specimen to form a beam spot and a detection optical system for detecting a reflection light ray originating from the beam spot comprises a storage unit in which the position and size of a standard particle of known size on a specimen for correction are stored in advance, a correction processing section which, when the correction specimen is used as an inspection target, correlates a detected scattering light quantity from the standard particle with the known size of standard particle stored at a corresponding position in the storage unit to prepare a correlation between the scattering light quantity and a true value, and a signal processing section which, when an inspection wafer is used as an inspection target, converts a detected scattering light quantity into a defect dimension.