1. A voice synthesizer, comprising: an emphasis degree deciding unit for extracting a word or a collocation to be emphasized from among respective words or respective collocations on the basis of an extracting reference with respect to the each word or the each collocation included in a sentence and deciding an emphasis degree of the extracted word or the extracted collocation; and an acoustic processing unit for synthesizing a voice having an emphasis degree that is decided by the emphasis degree deciding unit provided to the word to be emphasized or the collocation to be emphasized.