1. A decoding method of a Viterbi decoder, the Viterbi decoder having a branch metric unit, an add-compare-select unit and a path memory unit, the method comprising the steps of: performing a longitudinal arrangement with respect to a trellis diagram corresponding to the Viterbi decoder so as to obtain a longitudinal arrangement trellis diagram; processing the longitudinal arrangement trellis diagram according to a run length limited (RLL) code so as to obtain a RLL trellis diagram; computing a current input branch-metric value by the branch metric unit according to the RLL trellis diagram; obtaining a next state value by computing the current input branch-metric value, a next input branch-metric value and a current state by the add-compare-select unit so as to obtain a next state value; recording results computed by the add-compare-select unit in the path memory unit so as to obtain a survivor path; and decoding the recorded survivor path.