In this article, we will discuss the mathematics pedagogy in the school, Taihoku Kodo Gakko (Taipei High School), established in 1922-1945 with a student’s mathematics notebook that was used in 1944-45. The student’s mathematics « professor » was likely Kato Heizaemon (1891~1975), a researcher of wasan, which is Japanese mathematics in the Edo period. Due to Kato’s background, the contents in the notebook show characteristics of traditional Japanese mathematics. In this study, we will take the topics of the binomial theorem, polynomials, series and approximations as examples to explain how the teacher’s emphasis on numerical approximations reflects the need for practical purposes in mathematics teaching and learning. For instance, the topic of polynomials is devoted more to the methods of estimation and calculation than to the properties of algebraic operations.