2. The speech recognition system, according to claim 1 , further comprising: distance value buffers which store distance values generated by the distance calculation unit; and acoustic lookahead value buffers which store acoustic lookahead values generated by the acoustic lookahead unit, wherein in each of the distance value buffers, operations of writing the distance value from the distance calculation unit, reading out the distance value to the acoustic lookahead unit, and reading out the distance value to the word string matching unit are performed, in each of the acoustic lookahead value buffers, operations of writing the acoustic lookahead value from the acoustic lookahead unit and reading out the acoustic lookahead value to the word string matching unit are performed, wherein, at any point in time, the distance value buffer in which the distance value from the distance calculation unit is written, the distance value buffer from which the distance value to the acoustic lookahead unit is read out, and the distance value buffer from which the distance value to the word string matching unit are read out, are different from one another, and the acoustic lookahead value buffer in which the acoustic lookahead value from the acoustic lookahead unit is written and the acoustic lookahead value buffer from which the acoustic lookahead value to the word string matching unit is read out are different from each other.
2. The speech recognition system, according to claim 1 , further comprising: distance value buffers which store distance values generated by the distance calculation unit; and acoustic lookahead value buffers which store acoustic lookahead values generated by the acoustic lookahead unit, wherein in each of the distance value buffers, operations of writing the distance value from the distance calculation unit, reading out the distance value to the acoustic lookahead unit, and reading out the distance value to the word string matching unit are performed, in each of the acoustic lookahead value buffers, operations of writing the acoustic lookahead value from the acoustic lookahead unit and reading out the acoustic lookahead value to the word string matching unit are performed, wherein, at any point in time, the distance value buffer in which the distance value from the distance calculation unit is written, the distance value buffer from which the distance value to the acoustic lookahead unit is read out, and the distance value buffer from which the distance value to the word string matching unit are read out, are different from one another, and the acoustic lookahead value buffer in which the acoustic lookahead value from the acoustic lookahead unit is written and the acoustic lookahead value buffer from which the acoustic lookahead value to the word string matching unit is read out are different from each other. 4. The speech recognition system according to claim 2 , wherein the buffer management unit includes: a function of monitoring operations of the distance calculation unit, the acoustic lookahead unit and the word string matching unit; and on conditions that the distance calculation unit cannot write the distance values in the distance value buffers any more, the acoustic lookahead unit and the word string matching unit read out all of the distance values from the distance value buffers, the acoustic lookahead unit cannot write the acoustic lookahead values in the acoustic lookahead value buffers any more, and the word string matching unit read out all of the acoustic lookahead values from the acoustic lookahead value buffers, a function of operating: the distance value buffer used for writing by the distance calculation unit as a next one from which the acoustic lookahead unit reads out; the distance value buffer used for reading by the acoustic lookahead unit as a next one from which the word string matching unit reads out; the distance value buffer used for reading by the word string matching unit as a next one into which the distance calculation unit writes; the acoustic lookahead value buffer used for writing by the acoustic lookahead unit as a next one from which the word string matching unit reads out; and the acoustic lookahead value buffer used for reading by the word string matching unit as a next one into which the acoustic lookahead unit writes.