PURPOSE: To realize a low level of compressibility and also to simplify the algorism of compression, by setting the size picture element of a partial picture so as to minimize the ratio of gray compressed picture elements to all compressed picture elements.

CONSTITUTION: For instance, a picture element 14 contains 1680 horizontal picture elements and 2376 vertical picture elements. In case a partial picture 15 contains (8×8) picture elements, the picture information quantity Is (bit) is calculated as Is=8×8×62370=3991680 (bits). Then the area of a type character picture is totally regarded as a gray color in case 25 characters exist in a row on an average and in 30 rows in all with 24 picture elements for height H and 24 picture elements for pitch P in terms of the type character picture of a Japanese words 16. In such a case, the ratio α of gray compressed picture elements to all compressed picture elements is shown as α=24×24×25×30×1.3÷64÷ 623700.14, where 1.3 is set by estimating that the actual gray part increases about 30%. Therefore the information quantity Ic is calculated after compression as shown by an equation (1). This system calculates the compressibility R as R= 683615÷39916800.17 to obtain a large degree of compression.