1. A speech synthesis process comprising the steps of: (a) generating, for each of synthesis units for speech synthesis, a plurality of first data portions, each having a fixed period length, from a first speech data representing each of said synthesis units; (b) generating, for each of said synthesis units, the same number of second data portions as that of said first data portions, each of said second data portions corresponding acoustically to each of said first data portions, from at least one second speech data representing each of said synthesis units, said second speech data extending over a duration different from that of said first speech data; (c) determining a synthesis unit to be synthesized; (d) determining a target duration of said determined synthesis unit; (e) determining a period length of each of a series of synthesis frames, said series of synthesis frames extending over said determined target duration of said determined synthesis unit and comprising the same number of frames as that of said first data portions, by interpolation based on said determined target duration of said determined synthesis unit, with reference to each of period lengths of said first and second data portions for said determined synthesis unit, each of said first and second data portions corresponding to each of said synthesis frames; (f) determining synthesis parameters to each of said synthesis frames, by interpolation based on said determined target duration of said determined synthesis unit, with reference to each of synthesis parameters of said first and second data portions for said determined synthesis unit, each of said first and second data portions corresponding to each of said synthesis frames; and (g) synthesizing a speech based on said determined period length and synthesis parameters of each of said synthesis frames.