A defect inspection apparatus for inspecting defects on an inspecting object includes an illuminator which irradiates a beam of light on the inspecting object, a photo-detector which detects rays of light from the inspecting object due to the irradiation of the light beam by the illuminator, a defect detector which detects a defect by processing a signal obtained through detection by the photo-detector, a characteristic quantity calculator which calculates a characteristic quantity related to a size of the defect, and a defect size calculator which uses a relation between size and characteristic quantity which is calculated by an optical simulation and calculates a size of the detected defect.