This is a study of the problem of the transition from the two-word stage to the multiple-word stage in the course of language acquisition. The longitudinal speech data of six normal Japanese children were examined. The results showed a close relationship between the state of the knowledge of the projection of lexical category V and N and the appearance of multiple-word utterances. We proposed the hypothesis that it is the complete realization of the knowledge of the projection of lexical category V and N that enables children to break through the constraint on the length of utterances and makes them produce multiple-word utterances.