1. A Viterbi decoding apparatus for decoding input data, comprising: branch metric calculation means for calculating a branch metric for said input data for a plurality of time slots; state metric storage means for storing a state metric formed of an accumulation of sequentially calculated branch metrics by said branch metric calculation means; add-compare-select computation means for performing, in a parallel manner, add-compare-select computations in a current processing stage on said branch metric for the plurality of time slots calculated by said branch metric calculation means and on said state metric supplied from said state metric storage means and formed of the accumulation of branch metrics for a period up to a preceding processing stage at intervals of the plurality of time slots for each of a plurality of paths coming into one state node, thereby obtaining a maximum likelihood path which has a maximum likelihood of being closest to a code sequence of said input data; and maximum likelihood sequence decision means for decoding said input data according to a content of said path obtained by said add-compare-select computation means at an interval of the plurality of time slots, wherein said add-compare-select computation means includes: a plurality of adding means corresponding to said plurality of paths for adding said branch metric for the plurality of time slots calculated by said branch metric calculation means in the current processing stage and said state metric for the period up to the preceding processing stage stored in said state metric storage means for respective paths of said plurality of paths at intervals of the plurality of time slots and for outputting a plurality of added sums; a plurality of comparison means for comparing each of said plurality of added sums with each other added sum and outputting a plurality of higher likelihood sums representing added sums having a higher likelihood than the other added sums based on the comparison; and selection means for selecting the maximum likelihood path which has the maximum likelihood according to results of the comparison made by said plurality of comparison means, wherein said selection means generates a first selection signal supplied to said state metric storage means and a second selection signal supplied to said maximum likelihood decision means.