A soft demodulation method and apparatus including calculating partial sums for a unit of each predetermined number of bits of a codeword received from a channel; calculating a value of each entry of the decoding table by referring to the partial sums; and detecting a maximum among values of all entries of the decoding table and calculating a log-likelihood ratio (LLR) using the detected maximum. Accordingly, it is possible to reduce the amount of computation required to perform a soft demodulation process using run-length limited (RLL) codes and to simplify the soft demodulation process.