1. A speech processing device for processing an input utterance and registering a word contained in the input utterance based on the processing result, comprising: recognition means for recognizing a continuous input utterance; unknown word determination means for determining whether or not a result of recognition by the recognition means contains an unknown word; network generating means for generating a network having paths including sub-words at times corresponding to word boundaries on both sides of the unknown word and a path not including the sub-words if the unknown word determination means determines that the result of recognition by the recognition means contains the unknown word; acquiring means for acquiring a word corresponding to the unknown word if the unknown word determination means determines that the result of recognition by the recognition means contains the unknown word; and registering means for registering the word acquired by the acquiring means while associating the word with other information.