PURPOSE: To simplify the circuit constitution and to improve the processing speed by comparing the standard white digital data weighted corresponding to the threshold of a dither pattern and the digital data of the read image and obtaining the binarizing image signal.

CONSTITUTION: From a main control circuit 7, a CCD sensor 1 reads a standard white pattern signal, and stores digital data for 1 line through an A/D converter 2 to a RAM 4. A large image signal read by the CCD sensor 1 is A/D-converted, inputted into a comparator 6, synchronized with this, the standard white pattern of the same picture element is read from the RAM 4, compared with the digital data to perform the weighting in correspondence to the threshold of a dither pattern set beforehand by a multiplier 5, and the shading correcting processing and the binarizing processing are performed. Thus, a memory can be decreased, the circuit constitution is simplified, the cost is reduced and the processing speed can be improved.