PURPOSE: To attain binarization for an image in which missing of a thin line or unsharpened edge is not in existence.

CONSTITUTION: A picture element discrimination section 11 uses an intermediate tone binarization section 12 being a 1st binarization means to binarize an input image when the picture element level of the input image is within a range between two threshold levels T0 and T1. Furthermore, when the picture element level of the input image is at the outside of the range, a binarization section 13 being a 2nd binarization means executes the binarization.