Abstract: A viterbi decoder includes a maximum branch metric value detecting device separated from a feedback loop consisting of an ACS arithmetic unit, a normalization arithmetic unit, and a state value storing device. In operation, a maximum branch metric value is detected by utilizing the output of the state value storing device, to thereby enhance operating speed. Further, a judgment is made as to whether a state having the minimum state value at an immediately preceding time is back-traceable from a state having the minimum state value at a current time by using survivor path information. Non-back-traceable non-correspondences are then detected, and the number of occurrences of non-correspondences are counted. If the occurrences of non-correspondences exceed a reference value, it is judged a non-synchronization. In another method, synchronization/non-synchronization is judged by detecting whether a trace back is possible, without utilizing survivor path information.