1. A speech processing apparatus for processing an input utterance and registering an unknown word contained in the input utterance into a dictionary database on the basis of the processing result, comprising: recognition means for recognizing the input utterance; unknown word determination means for determining whether the recognition result of the input utterance obtained by the recognition means contains an unknown word on the basis of an acoustic model representing acoustic features of individual phonemes and syllables of a language; recognition result rejection means for determining whether the recognition result determined by the unknown word determination means to contain an unknown word is rejected or not for acquisition and for registering into the dictionary database; and word extracting means for registering and for acquiring pronunciation of a word corresponding to the unknown word contained in the recognition result determined not to be rejected by the recognition result rejection means, wherein the acquired pronunciation of the word is used for subsequent speech recognition, wherein the recognition result rejection means determines whether the recognition result is rejected or not on the basis of a confidence measure for a known word immediately before the unknown word and a confidence measure for a known word immediately after the unknown word contained in the recognition result.