This article reports on the development and use of the Developmental Sentence Scoring for Japanese (DSSJ), a new morpho-syntactical measure for Japanese constructed after the model of Lee's English Developmental Sentence Scoring model. Using this measure, the authors calculated DSSJ scores for 84 children divided into six age groups between 2; 8 and 5; 2 on the basis of 100-sentence samples collected from free-play child-adult conversations. The analysis showed a high correlation of the DSSJ overall score with the Mean Length of Utterance. The analysis of the DSSJ sub-area scores revealed large variations between these sub-area scores for children with similar overall DSSJ scores. When investigating the high-scoring children (over 1 SD over group average), most children scored high in three to five sub-areas, but the combination of scores for these sub-areas varied from child to child. It is concluded that DSSJ is a valuable tool especially for language acquisition research. The overall DSSJ score reliably reflects the overall morpho-syntactic development of Japanese children, and the sub-area scores provide specific information on individual acquisition patterns.