Abstract: In the Anthropocene, the occurrence of ‘new plastic formations’ is being reported at locations around the globe, but there have been few reports from coasts along the Asian side of the Pacific Ocean. In this study, the cases of ‘new plastic formations’, consisting mainly of ‘pyroplastics’, which are drifted on the northwestern coast of Japan’s Seto Inland Sea, were described. In Japan, the problem of harmful substances caused by ‘hamayaki’ (burning of drifted coastal debris at the shore) has been studied from an early stage as the problem of coastal marine plastic pollution. In addition, in the Japan Sea and Yellow Sea, drifting marine litter created via combustion has been studied, and potential amounts of ‘pyroplastic’ were suggested. ‘Pyroplastics’ has been identified as the source of microplastics due to those vulnerabilities, and easy degradation into smaller particles was verified in the ‘pyroplastic’ collected.