1. An emotion recognition apparatus that recognizes an emotion of a speaker from an input speech, comprising: a non-transitory memory device configured to store a program that is executable by a processor, which causes the emotion recognition apparatus to operate as the following: a characteristic tone detection unit configured to detect, from a plurality of characteristic tones, a characteristic tone that corresponds to the input speech and relates to a specific emotion; a speech recognition unit configured to recognize types of phonemes included in the input speech, based on the characteristic tone detected by the characteristic tone detection unit; a characteristic tone occurrence indicator computation unit configured to compute, for each of the characteristic tones, a characteristic tone occurrence indicator for each of the phonemes, based on the types of the phonemes recognized by the speech recognition unit, in accordance with a rule which predetermines a relationship between a position of each of the phonemes and the characteristic tone occurrence indicator indicating a likelihood of the phoneme being spoken with the characteristic tone at the position; and an emotion judgment unit configured to (i) judge a type of the emotion of the speaker in a phoneme at which the characteristic tone occurs in the input speech, from the characteristic tone occurrence indicator computed by the characteristic tone occurrence indicator computation unit and a type of the characteristic tone detected by the characteristic tone detection unit, in accordance with a criterion for judging the type of the emotion based on a combination of a type of the characteristic tone and the characteristic tone occurrence indicator for each phoneme and an adjacent one of the phonemes, and to (ii) judge a strength of the emotion of the speaker in the phoneme at which the characteristic tone occurs in the input speech, using the characteristic tone occurrence indicator computed by the characteristic tone occurrence indicator computing unit, in accordance with a rule that indicates for each type of the emotion that the emotion is high in strength when the characteristic tone occurrence indicator corresponding to the type of the emotion is in a first range, the emotion is medium in strength when the characteristic tone occurrence indicator is in a second range higher than the first range, and the emotion is weak in strength when the characteristic tone occurrence indicator is in a third range higher than the second range, wherein the strength of the emotion of the speaker determined by the characteristic tone occurrence indicator based on a frequency of occurrence of the phoneme in the input speech for which the corresponding characteristic tone is recognized.
2. The emotion recognition apparatus according to claim 1 , wherein the emotion strength judgment unit is configured to judge, using a characteristic tone occurrence indicator of a phoneme corresponding to a time position at which the characteristic tone detected by the characteristic tone detection unit occurs in the input speech, the strength of the specific emotion of the speaker in the phoneme at which the characteristic tone occurs, in accordance with the rule that indicates that the emotion is high in strength when the characteristic tone occurrence indicator is in the first range, the emotion is medium in strength when the characteristic tone occurrence indicator is in the second range, and the emotion is weak in strength when the characteristic tone occurrence indicator is in the third range, the characteristic tone occurrence indicator being included in the characteristic tone occurrence indicator computed for each of the phonemes, which corresponds to the characteristic tone detected by the characteristic tone detection unit, among a plurality of characteristic tone occurrence indicators computed by the characteristic tone occurrence indicator computation unit.