1. A singing voice-synthesizing method comprising: inputting phonetic unit information representative of a phonetic unit, time information representative of a singing-starting time point, and singing length information representative of a singing length, in timing earlier than the singing-starting time point, for a singing phonetic unit including a sequence of a first phoneme and a second phoneme; generating a phonetic unit transition time length formed by a generation time length of the first phoneme and a generation time length of the second phoneme, based on the inputted phonetic unit information; determining a singing-starting time point and a singing duration time of the first phoneme and a singing-starting time point and a singing duration time of the second phoneme, based on the generated phonetic unit transition time length, the inputted time information and singing length information; and starting generation of a first singing voice and a second singing voice formed by the first phoneme and the second phoneme at the singing-starting time point of the first phoneme and the singing-starting time point of the second phoneme, respectively, and continuing generation of the first singing voice and the second singing voice for the singing duration time of the first phoneme and the singing duration time of the second phoneme, respectively.