1. An error-correcting decoder which performs Viterbi-decoding for an input digital signal by using a Viterbi algorithm, comprising: flag signal adding means for adding a flag signal to a first location where a difference between path metrics in the Viterbi algorithm is lower than a threshold, and for continuously adding flag signals to locations preceding the first location as determined by back tracing; and a block-code decoder for block-code decoding the Viterbi-decoded signal, by regarding the locations flagged by the flag signal adding means as erasure locations.