In optical dark field defect inspection, the present invention provides including: condensing laser emitted from a light source in a line shape; reflecting the laser, with a mirror; irradiating the reflected laser via an objective lens to a sample placed on a table from a vertical direction; condensing reflected scattered light from the sample with the objective lens; shielding diffraction light occurred from a periodical pattern formed on the sample, in the reflected scattered light from the sample and scattered light occurred from the mirror, with a spatial filter; receiving the reflected scattered light from the sample, not shielded with the spatial filter, with an imaging lens, and forming an image of the reflected scattered light; detecting the image of the reflected scattered light; and processing a detection signal obtained by detecting the image of the reflected scattered light and detecting a defect on the sample.