1. A speech synthesizer comprising: a processor; an analyzer that performs a text analysis of an input document and extracts 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 candidates of a speech unit string 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 statistical model of prosody information of the speech unit included in the selected candidates, for each speech unit; a second estimator that re-estimates prosody information that maximizes a third likelihood by differentiating the third likelihood with respect to a parameter of the second prosody model, the third likelihood being calculated by linearly coupling the first likelihood and a second likelihood representing probability of the second prosody model; and a synthesizer that generates synthetic speech by concatenating the speech units included in the selected candidates on the basis of the prosody information estimated by the second estimator, wherein the processor executes at least one of the analyzer, the first estimator, the selector, the generator, the second estimator, and the synthesizer.