This study proposes a lesson plan for learning to understand developers’ design intentions of familiar products through disassembly and assembly activities in technology education at junior high school. Lessons were planned and implemented, and the educational effect was verified. As a framework for disassembly and assembly learning, the following lesson modules were conceived: “understanding of products,” “disassembly and assembly activities,” and “analysis and evaluation of products.” Learning materials were selected to correspond to the learning activities, including the Dyson Engineering Box, which is a product model of a vacuum cleaner that can easily be used as a teaching material in disassembly and assembly activities. The educational materials can be freely borrowed by all junior high schools. The results of the study in 60 second-grade junior high school students show that more than 90% were able to think about the optimization of the technology incorporated in the vacuum cleaner. In addition, more than 80% of respondents to the questionnaire survey answered positively that they could understand the structure and mechanism of the vacuum cleaner after learning. The students’ free description responses included “disassembly and assembly of products,” “problem solving,” “creating ideas,” and “design and manufacture.” In conclusion, students were able to understand the process of problem solving in familiar life and society using designs that lead to optimal solutions from multifaceted design elements. Through this practice, students can understand the developer’s design intentions. This lesson plan is proposed for students’ learning leading to future technical education.