Disclosed is an apparatus having a light source of a deep ultraviolet ray for detecting a small foreign matter or pattern defect, which may arise during a process for manufacturing a semiconductor device or the like, in high resolution. The apparatus comprises a means for detecting a damage on an optical system due to a wavelength reduction thereby to save a damaged portion, and a means for comparing an optical system arrangement with that at the manufacturing time and detecting the abnormality thereof, to thereby make a correction, so that the apparatus can inspect the defect on an object substrate stably at a high speed and in high sensitivity. Also disclosed is a method for the stable inspection. The apparatus is provided, in the optical path of the optical system, with a means for detecting the intensity and the convergent state of an illumination light, and a means for detecting the abnormality of the optics system and for saving an abnormal portion from alignment with an optical axis. The apparatus is constituted such that the optical system is adjusted to make corrections for the optical conditions at the manufacturing time, thereby to elongate the lifetime of the optical system in the inspecting apparatus and to detect the small defect stably.