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.