1. A speech synthesizer comprising: an analyzer that performs a text analysis of an input document and extract a linguistic feature used for prosody control; a first estimator that selects a first prosody model adapted to the extracted linguistic feature from predetermined first prosody models that are models of speech prosody information and that estimates prosody information that maximizes a first likelihood representing probability of the selected first prosody model; a selector that selects, from a speech unit storage storing speech units, a plurality of speech units that minimizes a cost function determined in accordance with the prosody information estimated by the first estimator; a generator that generates a second prosody model that is a model of prosody information of the selected speech units; a second estimator that estimates prosody information that maximizes a third likelihood calculated on the basis of the first likelihood and a second likelihood representing probability of the second prosody model; and a synthesizer that generates synthetic speech by concatenating the selected speech units on the basis of the prosody information estimated by the second estimator.