A city is not sustainable where a lot of trash is dumped. In order to make a city sustainable, we must reduce the amount of trash. What has become a big problem in Japan in recent years results from plastic bags such as supermarket checkout bags. These bags are light, strong and waterproof enough to carry goods we have bought without any stain. Ironically, however, plastic bags are so durable and easy to use that they have led to serious environmental pollution and huge waste of petroleum. As a countermeasure to these problems, in Toyama Prefecture, some supermarkets started charging extra fees on plastic bags in 2008. In 2009, the students at our school in those days conducted a research on how many plastic bags each household used. The research said that the amount of the bags each family used per week was around half as much as those in another prefecture. Hence they concluded that charging some fee on a plastic bag is somehow effective to reduce the amount of those bags. Today, ten years after this study, our new research shows that the amount has dramatically reduced to fewer than 10. While the charging has undoubtedly contributed to this outcome, is there any other factor for reducing the amount? If there is, we assume, we can use it to further decrease not only the usage of plastic bags but also other kinds of trash, leading to a sustainable living in a big city.