1. A speech recognition system, comprising: a distance calculation unit which generates a distance value between speech features, inputted sequentially, and each acoustic model; an acoustic lookahead unit which generates an acoustic lookahead value by using the distance value previously generated by the distance calculation unit, in parallel with generation of the distance value by the distance calculation unit; a word string matching unit which performs word matching by using the distance value previously generated by the distance calculation unit and the acoustic lookahead value previously generated by the acoustic lookahead unit to thereby generate a recognition result, in parallel with generation of the distance value by the distance calculation unit and generation of the acoustic lookahead value by the acoustic lookahead unit.