Abstract: This specification discloses an image scanning system in which an object image formed by an optical system is electrically scanned by utilization of a photosensor array device and the image scanning output then obtained is quantized to thereby provide quantized image data. According to the improvement of the present invention, there are obtained two different quantized image data, namely, first quatized image data quantized by a voltage level determined on the basis of the image scanning output from the sensor device and second quantized image data quantized by a predetermined fixed voltage level.