PURPOSE: To improve a production yield by comparing a detection signal of the scattered light of action surface with that of back surface through the laser beam scanning of the action surface side, thereby deciding the flaw of foreign matter, etc., outputting a flaw signal, and disregarding the foreign matter, etc., of the back surface.

CONSTITUTION: When a laser spot is scanned, the light receiving signals of light receiver 3b and 3b' receiving scattered light are detected with the same proper level threshold and each detection signal is obtained in signal processing circuits 4a and 4d, and each detecting signal is inputted to a microprocessor 4d. In the microprocessor 4d, the position of an unit cell corresponding to the scanned spot is recognized, detection signal data with respect to each unit cell is stored in address corresponding to the memory 4e. The scanning of the front and back surface sides with respect to one scanning line in a X- direction is finished, so that data corresponding to the same unit cell is read out and compared successively and the flaw of the foreign matter is judged. Thus, the yield is improved.