To perform the bit-rounding of the input data with no dependence of the bit series of the input data and before the data compression processing, without deteriorating the efficiency of the data compression processing that makes use of the time or space correlation.

A bit-rounding processing part 15 performs the bit-rounding of the input data with no dependence on the bit series of the input data, based on the pseudo random number produced at a pseudo-random number generation part 14. Then a compression part 16 performs the data compression processing to the output data of the part 15 by making use of the time or space correlation. The part 14 is initialized at the timing, when the head data of each frame are inputted to the part 15 by the initialization pulses received from the timing generation parts 19 and 20. Therefore, the same series of pseudo-random numbers are secured in all frames.