PURPOSE: To form a clear picture by optimalizing picture elements in a picture-element block after a 2×2 picture-element block of an original picture is converted into a 3×3 picture-element block, by using relations between respective picture elements of the original picture and its adjacent picture elements.

CONSTITUTION: Code converter 53 determines picture-element levels of four corners Q1, O3, O7, and O9 of a converted picture-element block obtained by converting a 2×2 original picture-element block into a 3×3 picture-element block, and also determines number nB of black picture elements. If nB is 1∼4, code converter 54 determines black picture elements according to the number of middle points O2, O4, O6, and O8. When the number does not reach nB at the determination of black picture elements as to middle points Q2, O4, O6, and O8, converter 54 supplies the remaining number nB' of black picture elements to code converter 55. Consequently, converter 55 regards center point O5 as black and feeds number nB'-1 back to converter 54, which when nB'-1≠0, considers some of the determined middle points to be black picture elements to optimalizing the white or black relations among all picture elements, thereby forming a clear picture.