1. A defect inspection device comprising: an irradiating unit, the irradiating unit comprising: an illumination light adjusting unit which adjusts light emitted from a light source into illumination light having predetermined irradiation conditions; and an illumination intensity distribution control unit which controls an illumination intensity so that an illumination intensity in a predetermined detection target area out of an illumination area on a surface of a sample on which the illumination light is irradiated is 50% or more of an illumination intensity at a center position of the illumination light on a surface of the sample and an illumination intensity in an illumination area other than the predetermined detection target area is 0.1% or less of an illumination intensity at a center position of the illumination light on a surface of the sample; a scanning unit which scans the sample in a direction perpendicular to a longitudinal direction of the illumination area in the irradiating unit; a detecting unit which detects scattered light generated from a surface of the sample due to illumination light irradiated by the irradiating unit; and a determining unit, the determining unit comprising: a defect presence/absence determining unit which processes a detection signal based on scattered light from a surface of the sample detected by the detecting unit and determines presence/absence of a defect on a surface of the sample; and a defect dimension determining unit which determines, when presence of a defect is determined by the defect presence/absence determining unit, a dimension of the defect.