An original of copying, fax, or the like is read by a CCD image sensor and resulting image data are input to a data abnormality detecting section via an A/D converter and correcting unit. A maximum value and a minimum value of the pieces of gradation information of the image data are detected and compared with a black determination value and a white determination value, respectively, whereby whether the original is all white or all black is detected and an data abnormality announcing section issues an alarm message or the like. The data abnormality detection is performed before a binarizing section binarizes the image data.

 
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