1. A sentence generating apparatus comprising: recording means for storing a plurality of idioms comprising a verb and having an intrinsic meaning, a plurality of words, a list of related words of said words, word classes of said words, and phonetics of said words; first word selecting means for reading out a first idiom from said recording means and for selecting a word contained in the first idiom as a first word; second word selecting means for reading out a word stored in said recording means and having a pronunciation that is the same as or analogous to said first word and being of the same word class as said first word, based on the phonetics and the word class of the recorded first word, and for selecting the read-out word as a second word; third word selecting means for reading out a next word stored in said recording means and having a semantic relationship with the second word, and selecting the next word as a third word, wherein the third word selecting means determines a semantic relationship of the next word with the second word based on a common word in the list of related words of the next word and the list of relating words of the second word stored in the recording means, and wherein, when no semantic relationship is found between the next word and the second word, the third word selecting means selects a second next word; idiom generating means for generating a second idiom by replacing the first word contained in the first idiom with the second word; and sentence generating means for generating a first sentence comprising the third word as a subject of the first sentence and the first idiom and for generating a second sentence comprising the third word as a subject of the second sentence and the second idiom.