In recent years, Yokohama City, Japan, has seen substantial efforts to address socio-environmental issues. One such effort is the G-30 program, which has helped reduce garbage waste production by 43 percent in 2010, since 2001. The eco-picture diary, an environmental education project, has been identified as contributing to the success of G-30. However, no research to date has examined the diary's effect on garbage reduction, nor accounted for what the project is and how it became prevalent. Using questionnaires with citizens and interviews with other stakeholders, this case study addresses these gaps. Key findings concern how people perceived the diary to have been central to garbage reduction by citizens. The study also shows how the diary illustrates an example of backcasting, ecopedagogy, reflective learning, and place-based approaches to environmental education, as well as how uptake of the project can be explained via diffusion of innovation theory and the theory of planned behavior.