The importance of deep understanding is well-known in education. Especially when students learn complex and difficult contents such as high school mathematics, it is hard to completely understand the knowledge only during classroom lessons. In such cases, we need to promote deep understanding throughout a learning cycle including after-class review. The purpose of this research was to develop homework tasks that can promote understanding of the reasoning behind mathematical formulae and/or theorems, to extract an effective way of task designing, and to examine feasibility in a real educational setting. The authors implemented the practice in a high school and placed emphasis on i) clarifying the definition of deep understanding for each lesson, ii) linking classroom lessons and homework in terms of how to promote students’ explanation, and iii) designing tasks/questionswhich can avoid rote memorization and/or copying. The analysis results of quantitative data suggest a positive effect from the intervention. However, for some students, the effectiveness and value of the intervention seemed not to be clear enough. The limitations of and future directions for this research are discussed.