1. A prosody generator, comprising: a data dividing unit implemented at least by a hardware including a processor and which divides into subspaces the data space of a learning database as an assembly of learning data indicative of feature quantities of speech waveforms; a density information extracting unit implemented at least by a hardware including a processor and which extracts density information indicative of a density state in terms of information quantity of the learning data in each of the subspaces divided by the data dividing unit, a prosody information generating method selecting unit implemented at least by a hardware including a processor and which selects either a first method or a second method as a prosody information generating method based on the density information, the first method involving generating the prosody information using a statistical technique, the second method involving generating the prosody information using rules based on heuristics, wherein the prosody information generating method selecting unit selects the second method when the density information indicates the density state is sparse; and an output unit which outputs a generated synthetic speech based on the prosody information.