A word dictionary including sets of a character string which constitutes a word, a phoneme sequence which constitutes pronunciation of the word and a part of speech of the word is referenced, an entered text is analyzed, the entered text is divided into one or more subtexts, a phoneme sequence and a part of speech sequence are generated for each subtext, the part of speech sequence of the subtext and a list of part of speech sequence are collated to determine whether the phonetic sound of the subtext is to be converted or not, and the phonetic sounds of the phoneme sequence in the subtext whose phonetic sounds are determined to be converted are converted.