PURPOSE: To attain effective data contraction, by utilizing the principles that a bit pattern being comparatively monority appears frequently, in the bit patterns of codes appeared in original data.

CONSTITUTION: Original data are 1-word 8-bit and they are 15 kinds as shown in Figure, from X(21) to Δ(23). Thus, most codes can be exprssed in codes of 4-bit per one word. Bit patterns in 4-bit have 16 types corresponding to hexadecimal number, in which "0" is taken as a specific code and other 15 kinds of bit patterns are taken as compression code in 4-bit. Then, the data contraction can be performed independently of the types of codes constituting the original data and the consecutive appearance of the same codes.