The purpose of this study is to clarify by what mechanism and from what kind of plastic products the secondary microplastics are generated from the perspective of polymer science and polymer engineering. Various approaches of us lead the following results. In the analysis using the pyrolysis method, individual component information was successfully extracted from the plastic mixture samples by means of Py-APGC-MS measurements with selecting the ions characteristic of the object polymer and recording the ion chromatogram with high mass resolution. In the morphological analysis of secondary microplastics, the microplastics recovered from rivers and beaches showed cracks as in the outdoor-exposed polymers, where light-induced oxidation takes place. On the other hand, no crack was observed in the microplastics recovered from marine. In a study considering microbial corrosion, the ”ABC degradation” mechanism was revealed: it was found that a complex degradation takes place where the hydrophilization of polymer proceeds owing to the photo-oxidative degradation (Abiotic) and microbial corrosion (Biotic), followed by the micro-flake detachment (Cutting) due to the dissociation of the hydrophilic zone caused by water ingress. Chemiluminescence analysis shows that hydrophobic polyolefins such as HDPE and PP tends to decelerate the degradation owing to the suppression of temperature increase and oxygen diffusion due to the presence of seawater. Focusing on mechanical effects, it is was found that fatigue may cause the fragmentation of plastic materials and that the jet mill could efficiently fine-grind plastics. In the future, a database of literature and research results on the formation mechanism of microplastics will be build. In addition, the microplastic formation behavior through long-term outdoor exposure will be observed and the microplastic accelerated generation test methods will be developed.