1. An arithmetic decoding method in which a context variable is initialized and arithmetic decoding is performed, using the context variable, on a binary string which corresponds to a value of a given variable, the context variable specifying a probability of a possible value of each of elements included in the binary string, the arithmetic decoding method comprising: determining an initialization method from among a plurality of initialization methods as a method of initializing the context variable, the initialization method corresponding to the given variable or a group which includes the given variable; and initializing the context variable using the determined initialization method.