1. A speech synthesis apparatus comprising: a language analyzer configured to analyze text data and output language information data that represents linguistic information of the text data; a statistical model storage configured to store a plurality of statistical models prepared by statistically modeling acoustic information included in speech; a model selector configured to select a statistical model from the plurality of statistical models based on the language information data; a parameter generator configured to generate a plurality of speech parameter sequences using the statistical model selected by the model selector; a basis model storage configured to store a basis model including a plurality of basis vectors, each of which expresses speech information for each limited frequency range; and a filter processor configured to output synthetic speech by executing filter processing of the plurality of speech parameter sequences and the basis model, wherein any of the plurality of speech parameter sequences represents weights to be applied to the basis vectors upon linearly combining the plurality of basis vectors.