1. A speech processing apparatus comprising: a speech storage configured to store a plurality of speech units of a conversion-source speaker and source-speaker attribute information corresponding to the speech units; a speech-unit extractor configured to divide the speech of a conversion-target speaker into a predetermined type of a speech unit to form target-speaker speech units; an attribute-information generator configured to generate target-speaker attribute information corresponding to the target-speaker speech units from the speech of the conversion-target speaker or linguistic information of the speech; a speech-unit selector configured to calculate costs on the target-speaker attribute information and the source-speaker attribute information using cost functions, and selects one or a plurality of speech units with the same phoneme from the speech storage according to the costs to form a source-speaker speech unit; and a voice-conversion-rule generator configured to generate speech conversion functions for converting the one or the plurality of source-speaker speech units to the target-speaker speech units based on the target-speaker speech units and the one or the plurality of source-speakerspeech units.
6. The apparatus according to claim 1 , wherein the voice-conversion-rule generator comprises: a speech-parameter extractor configured to extract target-speaker speech parameters indicative of the voice quality of the target-speaker speech units and source-speaker speech parameters indicative of the voice quality of the source-speaker speech units; and a regression analyzer configured to obtain a regression matrix for estimating the target-speaker speech parameters from the source-speaker speech parameters, the regression matrix being the voice conversion function.