PURPOSE: To obtain binary picture data by calculating an optimum threshold coincident with the visual sense characteristic of a human body an/d binarizing a multilevel picture data on the threshold.

CONSTITUTION: A binarized error calculation means 2 calculates a binarized error occurred by binarizing multilevel picture data as a square error for the respective thresholds. A degree of complexity calculation means 3 obtains the degree of complexity of a binary picture obtained by binarizing multilevel picture data for the respective thresholds. An optimum threshold calculation means 4 calculates the threshold whose square error becomes minimum and the threshold and whose degree of complexity becomes minimum and calculates a complexity minimum threshold nearest to the minimum threshold of the square error as the optimum threshold from the both values. Multilevel picture data is binarized to binary picture data by the threshold. Thus, the occurrence of noise and the notches of a boundary part can be prevented in a character picture and a graphic picture after a binarization processing, and the binary picture of high quality can be obtained.