The purpose of this paper is to clarify functions of a hierarchical classification of quadrilaterals in Japanese textbooks. From the perspective of the five functions of a hierarchical classification of quadrilaterals (de Villiers, 1994), geometry in the latest Japanese lower secondary school textbooks was analysed. While the three functions related to problem solving were identified, the other two functions related to exclusive and alternative definitions were not identified in the textbooks. The results of analysis imply that there is a limitation that contradictory definitions must not be presented in a textbook, and that textbooks can present only a part of defining activities.