1. An apparatus for coding a speech signal, comprising: an input device that inputs a mixed speech signal of a plurality of speakers; a separating device that analyzes period characteristics of the input mixed speech signal entered by said input device, and separates the input mixed speech signal into a plurality of single speech signals each associated with a corresponding one of the plurality of speakers, based on a result of the analysis; a first extracting device that extracts source speech characteristic parameters included in each of the single speech signals derived by said separating device, said source speech characteristic parameters representing characteristics of source speech generated from vocal cords of each of the speakers; a second extracting device that extracts a generic vocal-tract characteristic parameter from the input mixed speech signal, said generic vocal-tract characteristic parameter representing a vocal-tract characteristic shared by the plurality of speakers; and an output device that outputs the source speech characteristic parameters extracted by said first extracting device, and the vocal-tract characteristic parameter extracted by said second extracting device.