In defect scanning carried out in a process of manufacturing a semiconductor or the like, a light detection optical system comprising a plurality of photosensors is used for detecting scattered light reflected from a sample. The photosensors used for detecting the quantity of weak background scattered light include a photon counting type photosensor having few pixels whereas the photosensors used for detecting the quantity of strong background scattered light include a photon counting type photosensor having many pixels or an analog photosensor. In addition, nonlinearity caused by the use of the photon counting type photosensor as nonlinearity of detection strength of defect scattered light is corrected in order to correct a detection signal of the defect scattered light.