To provide a Viterbi-decoding method for reducing the storage capacity of a memory for tracing back.

This Viterbi-decoding method is used to update a route metric, according to an inputted data and to trace back. The trace-back processing is performed each time when a metric to be updated is updated, at terminating candidate positions 1, 2, and 3 of trace transition. Then, the trace-back processing stating from the position 1 is carried out from the position 1 to the initial position, and that starting from the position 2 or later is carried out from the position 2, to the position 1 as well as to position, where a constant k (k>5L: and L is constant length) is added to the position 1. The trace- back processing, starting from the position 3 is performed in a manner same to that starting from the position 2.