PURPOSE: To suppress the increase of redundant digits due to the propagation control of carry by reducing the compression ratio of a section due to the propagation control of carry and expanding the width of the section when the carry is to be propagated for a long period.

CONSTITUTION: Code digits Wi inputted from an input terminal 201 are successively inputted to a register 202 storing expression digits W1, W2. An output from an Xi generator 203 is inputted to a probability parameter generator 204 to find out the maximum digit Xi out of Y satisfying accumulation appearance ratio Ci(Y)≤(W1+W2)/B as the final output. Consequently, the values of W and B are changed and stored in registers 202, 206. On the other hand, the contents of a register F1 207 are supplied to an alarm circuit 209, and when carry transmission longer than M digits may be generated, the alarm circuit 209 sends a carry alarm signal to a section compensating circuit 210, all the contents of a register B 206 are turned to (b-1) and the contents of registers 202, 206∼208 are shifted to higher digits.