An apparatus can discriminate defects such as stains on bank notes by an optical scanner, A/D converter and comparing device. The apparatus comprises an optical scanner for projecting scanning light toward a detection area of the bank note which is being conveyed through the optical scanning device; a photoelectric converter for converting an optically scanned signal from the optical scanner into an electric analog signal whose level is substantially in proportion to the level of the optically scanned signal; an analog/digital converter for converting the electric analog signal into a digital signal; a timing control device for applying sampling pulses to the analog/digital converter so as to produce a sampled digital value from the analog/digital converter; a storing device for storing at least a presettable value which is used to discriminate the bank notes; and an arithmetic operation device for performing the arithmetic operation by introducing the sampled digital value and the presettable value and the presettable value so as to discriminate defects in the detection area of the bank notes.