A defect inspection method includes: illuminating an area on surface of a specimen as a test object under a specified illumination condition; scanning a specimen to translate and rotate the specimen; detecting scattering lights to separate each of scattering lights scattered in different directions from the illuminated area on the specimen into pixels to be detected according to a scan direction at the scanning a specimen and a direction approximately orthogonal to the scan direction; and processing to perform an addition process on each of scattering lights that are detected at the step and scatter approximately in the same direction from approximately the same area of the specimen, determine presence or absence of a defect based on scattering light treated by the addition process, and compute a size of the determined defect using at least one of the scattering lights corresponding to the determined defect.