1. A speech processing apparatus, comprising: a segmenting unit configured to divide a fundamental frequency of a speech signal corresponding to an input text into a plurality of pitch segments, based on an alignment between character strings of each linguistic level included in the input text and the speech signal; a parameterizing unit configured to generate a parametric representation of the pitch segments by means of a predetermined invertible operator such as a linear transform, and generates a group of first parameters in correspondence with the linguistic level; a descriptor generating unit configured to generate a descriptor which consists of a set of features describing the character strings, for each of the character strings in the linguistic level included in the input text; a model learning unit configured to classify the first parameters of the linguistic level of all the speech signal in the database into clusters based on the descriptor corresponding to the linguistic level, and learns for each of the clusters a pitch segment model for the linguistic level; and a storage unit configured to store the pitch segment models for each linguistic level together with the mapping rules between the descriptors describing the features of the character strings for the linguistic level, and the pitch segment models.