5. An apparatus as claimed in claim 1, further comprising: a source speech decoder that decodes source speech signals of the respective speakers, based on the source speech characteristic parameters extracted by said first extracting device with respect to the plurality of speakers, respectively, and forms a source speech signal for the plurality of speakers by synthesizing the decoded source speech signals of the respective speakers; a vocal-tract filter that filters the source speech signal for the plurality of speakers formed by said source speed decoder, based on the generic vocal-tract characteristic parameter extracted by said second extracting device, so as to decode a mixed speech signal indicative of mixed speech of the plurality of speakers; an error detector that detects an error between the mixed speech signal decoded by said vocal-tract filter and the input mixed speech signal; wherein said first extracting device extracts one of said source speech characteristic parameters so as to minimize the error detected by said error detector.
5. An apparatus as claimed in claim 1, further comprising: a source speech decoder that decodes source speech signals of the respective speakers, based on the source speech characteristic parameters extracted by said first extracting device with respect to the plurality of speakers, respectively, and forms a source speech signal for the plurality of speakers by synthesizing the decoded source speech signals of the respective speakers; a vocal-tract filter that filters the source speech signal for the plurality of speakers formed by said source speed decoder, based on the generic vocal-tract characteristic parameter extracted by said second extracting device, so as to decode a mixed speech signal indicative of mixed speech of the plurality of speakers; an error detector that detects an error between the mixed speech signal decoded by said vocal-tract filter and the input mixed speech signal; wherein said first extracting device extracts one of said source speech characteristic parameters so as to minimize the error detected by said error detector. 6. An apparatus as claimed in claim 5, wherein said second extracting device extracts a reflection coefficient as the vocal-tract characteristic parameter, said reflection coefficient being applied as a filter coefficient to said vocal-tract filter.