This paper proposes a novel spontaneous speech recognition approach to obtain not a whole utterance but reliably recognized partial segments of an utterance to achieve robust speech understanding. Our method obtains reliably recognized partial segments of an utterance by using both grammatical and n-gram based statistical language constraints cooperatively, and uses a robust parsing technique to apply the grammatical constraints. Through an experiment, it has been confirmed that the proposed method can recognize partial segments of an utterance with a higher reliability than conventional continuous speech recognition methods using an n-gram based statistical language model.