Abstract: A document image signal processor of gray level for use in a facsimile. The document image signal processor establishes a local window and a broad window from document image signals scanned by a CCD image sensor. An average level and a level difference between a maximum and a minimum level of the image signals are extracted from the local window. A maximum threshold of the background of the document image is extracted from the broad window. Thus, the image signals can be binarized with an adaptive threshold. Accordingly, the inventive image signal processor provides improved image, although the document image is collected (scanned) from a material whose brightness distribution is considerably ununiform.