A quality calculator apparatus for Viterbi-decoded data using zero-state metrics. The quality calculator for the Viterbi-decoded data includes: a Viterbi decoder which outputs zero-state metrics of input demodulated data according to the four possible transmission rates of FULL, HALF, QUARTER and 1/8; a register which stores respective zero-state metrics output from the Viterbi decoder; and a quality evaluation unit which reads the zero-state metrics stored in the register to evaluate the Viterbi-decoded data based on the zero-state metrics and which determines the actual transmission rate to be the one among the possible transmission rates which has the least zero-state metrics. The zero-state metrics of the demodulated data input to the Viterbi decoder are used as a quality evaluation parameter so that the quality evaluation can be correctly achieved to avoid errors in determining of the transmission rate, as compared to a quality calculator using a bit error ratio as the quality evaluation parameter.