5. The symbol insertion apparatus according to claim 1 , wherein for the symbol insertion likelihood obtained for the word sequence by a certain symbol insertion model, if the symbol insertion likelihood weighted according to the similarity between the speaking style feature of the word sequence and the plurality of speaking style feature models and the relevance between the symbol insertion model and the plurality of speaking style feature models is defined as symbol insertion likelihood for different models, the symbol insertion evaluation unit calculates, for each symbol to be inserted, a total of a predetermined number of the symbol insertion likelihood for different models selected from a plurality of the symbol insertion likelihood for different models which are obtained for the plurality of symbol insertion models, and evaluates the symbol with the largest total as an insertion symbol.
5. The symbol insertion apparatus according to claim 1 , wherein for the symbol insertion likelihood obtained for the word sequence by a certain symbol insertion model, if the symbol insertion likelihood weighted according to the similarity between the speaking style feature of the word sequence and the plurality of speaking style feature models and the relevance between the symbol insertion model and the plurality of speaking style feature models is defined as symbol insertion likelihood for different models, the symbol insertion evaluation unit calculates, for each symbol to be inserted, a total of a predetermined number of the symbol insertion likelihood for different models selected from a plurality of the symbol insertion likelihood for different models which are obtained for the plurality of symbol insertion models, and evaluates the symbol with the largest total as an insertion symbol. 6. The symbol insertion apparatus according to claim 5 , wherein the symbol insertion evaluation unit selects a predetermined number in order of high likelihood among the plurality of symbol insertion likelihood obtained for the plurality of symbol insertion models.