The present invention first provides adaptive binarization in which a binarizer outputs binary symbol in length, which is variable adaptively to the probability of the source. When the probability is low, it is desirable to decrease the length of the binary symbols to improve the efficiency of arithmetic coding and reduce the complexity of coding calculation. On the other hand, when the probability is high, it is desirable to increase the length of the binary symbols to improve the overall process speed of a decoder. Specifically, a binarizer, according to the present invention, binarizes mapping unit values from a non-binary symbol into binary symbols. The number of binary symbols is inversely proportional to the size of the mapping unit value. In the present invention, the mapping unit value is made variable adaptively to the probability. Thus, the number of binary symbols from the binarizer is also variable adaptively to the probability parameter.