1. A computer implemented prosodic control rule generation method executed on a suitably programmed computer, the method including: dividing an input text into language units; estimating a punctuation mark incidence at a boundary between the language units, the punctuation mark incidence indicating a degree that a punctuation mark occurs at the boundary, based on attribute information items of a plurality of language units adjacent to the boundary; and generating, by the computer, a prosodic control rule for speech synthesis including a condition for the punctuation mark incidence based on a plurality of learning data items each concerning prosody and including the punctuation mark incidence.