1. A voice synthesis device comprising: an utterance mode obtainment unit operable to obtain an utterance mode of a voice waveform for which voice synthesis is to be performed, the utterance mode being determined based on at least a type of emotion; a prosody generation unit operable to generate a prosody used when a language-processed text is uttered in the obtained utterance mode; a characteristic tone selection unit operable to select a characteristic tone based on the obtained utterance mode, the characteristic tone being observed when the language-processed text is uttered in the obtained utterance mode; a storage unit storing a rule, the rule being used for judging an ease of an occurrence of the selected characteristic tone based on a phoneme and a prosody; an utterance position decision unit operable to (i) judge whether or not each of a plurality of phonemes, of a phonologic sequence of the language-processed text, is to be uttered using the selected characteristic tone, the judgment being performed based on the phonologic sequence, the selected characteristic tone, the generated prosody, and the stored rule, and (ii) determine, based on the judgment, a phoneme which is an utterance position where the language-processed text is uttered using the selected characteristic tone; a waveform synthesis unit operable to generate the voice waveform based on the phonologic sequence, the generated prosody, and the determined utterance position, such that, in the voice waveform, the language-processed text is uttered in the obtained utterance mode and the language-processed text is uttered using the selected characteristic tone at the utterance position determined by said utterance position decision unit; and an occurrence frequency decision unit operable to determine a rate of occurrence of the selected characteristic tone, by which the language-processed text is uttered using the selected characteristic tone, wherein said utterance position decision unit is operable to (i) judge whether or not each of the plurality of phonemes, of the phonologic sequence of the language-processed text, is to be uttered using the selected characteristic tone, the judgment being performed based on the phonologic sequence, the selected characteristic tone, the generated prosody, the stored rule, and the determined rate of occurrence, and (ii) determine, based on the judgment, the phoneme which is the utterance position where the language-processed text is uttered using the selected characteristic tone, wherein said characteristic tone selection unit includes: an element tone storage unit storing (i) the utterance mode and (ii) a group of (ii-a) a plurality of characteristic tones and (ii-b) respective rates of occurrence by which the language-processed text is to be uttered using the plurality of the characteristic tones, such that the utterance mode is stored in correspondence with the group of the plurality of characteristic tones and the respective rates of occurrence; and a selection unit operable to select, from said element tone storage unit, the group of the plurality of characteristic tones and the respective rates of occurrence, wherein the selected group corresponds to the obtained utterance mode, wherein said utterance mode obtainment unit is further operable to obtain a strength of emotion, wherein said element tone storage unit stores (i) a group of the utterance mode and the strength of emotion and (ii) a group of (ii-a) the plurality of characteristic tones and (ii-b) the respective rates of occurrence by which the language-processed text is to be uttered using the plurality of characteristic tones, such that the group of the utterance mode and the strength of emotion is stored in correspondence with the group of the plurality of characteristic tones and the respective rates of occurrence, and wherein said selection unit is operable to select, from said element tone storage unit, the group of the plurality of characteristic tones and the respective rates of occurrence, the selected group corresponding to the group of the obtained utterance mode and the strength of emotion.