1. An adaptive maximum likelihood sequence estimator comprising: a transmission channel estimator for estimating, using partial sequences of sampled values of received signals and using estimated values of parts of sequences of transmission symbols, the impulse response of a transmission channel; and a Viterbi algorithm processor for sequentially estimating the sequences of transmission symbols from the estimated impulse response of the transmission channel and the partial sequences of sampled values, said Viterbi algorithm processor including a branch metric computer, an addition-comparison-selection portion, a path metric memory, and a path history memory; said branch metric computer including common element operation means for computing operational common elements which commonly appear in computations of plural branch values when all branch values for connecting between states are to be computed, in association with a regularity of coded alphabets of transmission symbols, branch value producing means for producing all branch values from the computed common elements, and branch metric final operation means for computing all branch metrics from the produced branch values.