PURPOSE: To obtain a binary image free from fine noise in the binarization processing method of a gradation image.

CONSTITUTION: A local threshold calculating circuit 11 calculates a local threshold from a threshold in a prescribed frame and outputs it to a comparator circuit 12. A representative value calculating circuit 16 fetches a noticed picture element Pi and vicinal picture elements a1 to a8 from a noticed point and vicinal picture element strage device 15 and calculates a representative value (a mean value, a middle value, etc.) of the density and outputs it to the comparator circuit 12. The comparator circuit 12 compares the inputted density representative value with the local threshold. The image is binarized by the comparison result outputted from the comparator circuit 12.