The purpose of this research is to consider children's "zone of proximal development" practically. In this research, focusing on "quantity per unit volume", "Dynamic Assessment" were practiced for six graders. Through the dynamic assessment in the whole class situation, 10 children's understanding of "quantity per unit volume" were proceeded, and their "zone of proximal development" were clarified. After that, focusing on 4 children, dynamic assessment were practiced individually. Consequently, they couldn't completely understand "quantity per unit volume", but they became to write the figures and tables, and their "zone of proximal development" were clarified.