Whereas Piaget established a conservation task to assess children's understanding of extensive quantity, this study aimed at establishing another kind of conservation task to assess children's understanding of the concept of intensive quantity, e.g. the thickness of paint on a board. The following study examined children's misconception by using our conservation task. Subjects were sixth graders and undergraduates. A typical question was asked in the conservation task : Which is thicker, the large part of a board or the small part, when it is painted uniformly? Although Subjects were taught in advance that the thickness of paint was given by its volume per unit area (1 cm^2) , commonly explained in school education, two-thirds of sixth graders and one-fourth of undergraduates failed to answer the question, suggesting they did not understand the concept of intensive quantity through the commonly given explanation. Results were discussed from the viewpoint of the method employed to teach the concept of intensive quantity.
