PURPOSE: To prevent coding efficiency from being reduced due to the approximate error of coding operation in an arithmetic coding method and to improve a compression ratio.

CONSTITUTION: A symbol appearance probability estimating means 50 selects one of n probability tables 52-1 to 52-n in accordance with the value of an augend (A) stored in an A register 62 included in an arithmetic code constituting means 60. The means 60 constitutes an arithmetic code by using the symbol appearance probability found out by the selected probability table as a coding parameter and outputs coded data. In the case of finding out an approximate value of A.PL obtained by multiplying the augend A by the appearance probability PL of a least power symbol(LPS), the value area [0.75, 1.5] of the A e.g. is divided into several sections, approximation is optimized in each divided section, improving the coding efficiency.