Users often abbreviate long words when using spoken dialogue systems, which results in automatic speech recognition (ASR) errors. We define abbreviated words as sub-words of an original word and add them to the ASR dictionary. The first problem we face is that proper nouns cannot be correctly segmented by general morphological analyzers, although long and compound words need to be segmented in agglutinative languages such as Japanese. The second is that, as vocabulary size increases, adding many abbreviated words degrades the ASR accuracy. We have developed two methods, (1) to segment words by using conjunction probabilities between characters, and (2) to adjust occurrence probabilities of generated abbreviated words on the basis of the following two cues: phonological similarities between the abbreviated and original words and frequencies of abbreviated words in Web documents. Our method improves ASR accuracy by 34.9 points for utterances containing abbreviated words without degrading the accuracy for utterances containing original words.