This study focuses on countering marine plastic litter (Setouchi Oceans X) in the watersheds and marine areas of four prefectures (Okayama, Hiroshima, Kagawa, and Ehime) in Setouchi. Outdated studies on the litter balance in this area necessitate a new assessment, specifically calculating the plastic waste balance. The study determined the current balance of plastic waste inflow, generation, and collection in the four prefectures.