PURPOSE: To obtain a binary image binarizing the both of a thick-density area and thin-density area without causing the thin-density area to disappear even in the case a gray level image where the thick-density area and the thin-density area coexist in an original image.

CONSTITUTION: In a binarization processing part 6, an edge image obtained through a space secondary differentiation processing part 2, a binarizing part 3 and a thinning part 4 is scanned at every picture element, and whether its scanned picture element corresponds to border line data or not is decided. In the case the scanned picture element corresponds to the border line data, binarization is executed by updating a threshold of a threshold memory 7, based on density data of the picture element in an original image having the same coordinate value as a coordinate value of the scanned picture element. On the other hand, in the case the scanned picture element does not correspond to the border line data, the binarization is executed, based on the threshold stored in the threshold memory 7 at the time point of scanning without updating the threshold of the threshold memory 7.