In this paper, we propose a method for automatically generating a pronunciation dictionary based on a pronunciation neural network that can predict plausible pronunciations (alternative pronunciations) from the canonical pronunciation. This method can generate multiple forms of alternative pronunciations using the pronunciation network for words that only occur a few times in the database and even for unseen words. Experimental results on spontaneous speech show that the automatically-derived pronunciation dictionaries give consistently higher recognition rates and require less computational time for recognition than a conventional dictionary.