1. A speech processing device, including a processor, 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 a result of recognition by the recognition means contains an unknown word; network generating means for generating a network having two kinds of paths when the unknown word determination means determines that the result of recognition by the recognition means contains the unknown word; wherein a first kind of path includes particular sub-words corresponding to word boundaries on one side or both sides of the unknown word, and wherein a second kind of path does not include the particular sub-words, wherein the particular sub-words are sub-words at positions where boundaries of the unknown words correspond to halfway points of the sub-words; acquiring means for acquiring a word corresponding to the unknown word when 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.