DECODING METHOD AND DECODING APPARATUS AS WELL AS PROGRAM

A decoding method for sorting received words in the order of the magnitude of the reliability of the received words, performing belief propagation using a parity check matrix diagonalized in the order to update the reliabilities, and repetitively performing the sorting and the belief propagation for the updated values, includes an inner repeated decoding process step of performing belief propagation using a parity check matrix diagonalized in an order of columns corresponding to symbols having comparatively low reliability values of the received words to update the reliability and repetitively performing the belief propagation based on the updated reliability; the inner repeated decoding process step in the second or later cycle of repetition thereof including diagonalization of the parity check matrix for restricted ones of the columns of the parity check matrix.