PURPOSE: To binarize only the target character or graphic in a simple circuit configuration without causing any crush and blur by deciding a density distribution of the subject image from the gradation area where the image data is detected and performing the binarization processing in each mode.

CONSTITUTION: A density detecting circuit 12 sorts the dynamic ranges of the image data into the prescribed gradation areas and then divided in the modes 1-4. A data correcting part 13 carries out the prescribed correction of data in response to the modes 1-4 and supplies the corrected data to a data selecting circuit 15 via a 5-bit conversion circuit 14. The circuit 15 selects one of those corrected data based on the mode deciding signal (m) sent from the circuit 12. The threshold value setting circuits 16 and 17 set the floating binary threshold value and the simple binary threshold value respectively. A Threshold value selecting circuit 21 selects the circuit 16 or 17 by the signal (m) to binarize the threshold value through a binarizing circuit 18. In such a way, the optimum threshold value can be automatically set and the satisfactory binarization processing is carried out.