1. A method of calculating a branch metric in Viterbi decoding for deriving a branch metric by comparing a codeword produced from a convolutional code with a sign of determination data obtained from conversion of received data for branch metric calculation, said method comprising the steps of: dividing said determination data into a sign and a metric and outputting them; determining whether said divided sign matches said codeword or not; selecting and outputting “0” when they match or selecting and outputting said metric when they do not match based on the determination result of the match or mismatch; and adding outputs selected when the signs match to outputs selected when the signs do not match to calculate a branch metric.