A circuit for converting a video signal into binary signals comprises a line image sensor for performing photoelectric conversion of an object to be read, to produce pixel signals, means for setting reference levels for respective sections of the line image sensor, the sections being divisions of the entire length of the line image sensor, and a comparator for comparing each pixel signal with the reference level for the section to which the pixel signal in question belongs, to produce a binary signal. The setting means includes means for producing digital reference level data and a D/A converter for converting the digital reference level data into an analog reference level signal.