1. An entropy coding method comprising: generating a reference matrix containing matrix components from an information matrix, the reference matrix being coded so as to have a correlation with the information matrix; classifying the matrix components within a range around an i-th row/j-th column component (where i and j are arbitrary integers) of the reference matrix to component sets with reference to distances from the i-th row/j-th column component, the component sets each containing matrix components equal in distance from the i-th row/j-th column component of the reference matrix; converting the matrix components belonging to the each component set into symbols; forming a context tree including a root node, a plurality of internal nodes corresponding to the component sets, and a plurality of branches which have one-to-one correspondence with the symbols and a plurality of leaf nodes; associating a sequence obtained by arranging the symbols in descending order of distance between the i-th row/j-th column component of the reference matrix and the matrix components with a path extending from the leaf node to the root node of the context tree, and calculating a coding probability of the i-th row/j-th column component of the information matrix as a weighting sum of probabilities which the respective nodes hold; and generating a codeword by arithmetically coding the i-th row/j-th column component of the information matrix in accordance with the coding probability.