PURPOSE: To enlarge a dynamic range with less quantity of bits, by performing a sub-instantaneous companding process on data words to be transmitted after digitizing after blocking them into every plural data words and, at the same time, performing a non-linear companding process on each word in the block.

CONSTITUTION: (n) Words out of data from an input terminal 1 are used for making one block and the maximum absolute value of the block is detected 2, and then, bit number information which can be compressed commonly to the whole words in the block corresponding to the absolute value is generated 3. Moreover, input data 1 are sub-instantaneously compressed by (n) words by means of compression information at a block unit adaptive processing circuit 5 through a block memory 4. Then each word is instantaneously compressed in terms of polygonal lines by means of a word unit adaptive processing circuit 6. Thereafter word data from an information generating circuit 3 and processing circuit 6 are converted into serial data by a multiplexer 7 and outputted 8. The outputted serial data are subjected to the polygonal line companding and sub- instantaneous companding, which is contrary to the above-mentioned processes, after the data are separated into each word at a receiving side. Therefore, a dynamic range can be enlarged with less quantity of bits.