10. The voice retrieval method according to claim 8 , wherein: in the time length obtaining step, the continuous time length for each of the phonemes contained in the phoneme string converted in the converting step is obtained for each phoneme state; and in the time length changing step, the continuous time length obtained for each of the phoneme states in the time length obtaining step is changed at a change rate defined for each phoneme state.
10. The voice retrieval method according to claim 8 , wherein: in the time length obtaining step, the continuous time length for each of the phonemes contained in the phoneme string converted in the converting step is obtained for each phoneme state; and in the time length changing step, the continuous time length obtained for each of the phoneme states in the time length obtaining step is changed at a change rate defined for each phoneme state. 11. The voice retrieval method according to claim 10 , wherein in the time length changing step, the continuous time length of a state that includes a start of phoneme utterance or an end of phoneme utterance in each phoneme state is changed at a smaller change rate than a change rate for the continuous time length of a state that does not include the start of phoneme utterance or the end of phoneme utterance.