1. A sentence generating apparatus comprising: recording means for having a plurality of idioms, a plurality of words, the meanings of said words, word classes of said words, and phonetics of said words; selecting means for reading out a preset idiom from said recording means and for selecting a preset word contained in the read-out idiom as a first word; first readout means for selecting, from said recording means, a word having the pronunciation same as or analogous to said selected first word and being of the same word class as said selected first word, based on the phonetics and the word class of the recorded first word, and for reading out the selected word as a second word; second readout means for selecting a word, determined to have a semantic relationship with the read-out second word, from said recording means, based on the meaning of the recorded second word, and reading out the selected word as a third word; idiom generating means for generating an idiom corresponding to the read-out idiom in which said selected first word among the words making up the read-out idiom is replaced by said read-out second word; and sentence generating means for generating a first sentence, composed of said third word and the read-out idiom, with the read-out third word as a subject of the sentence, and for generating a second sentence, composed of said third word and the generated idiom, with the read-out third word as a subject of the sentence.