Marine pollution by microplastics among plastic litter is an issue that should be addressed by the international community. In order to properly implement effective measures, it is necessary to understand the distribution of microplastics in the seas from land-based sources. In the "Guidelines for River Microplastic Monitoring Methods" published by the Ministry of the Environment (MOEJ) in June 2021, survey method is set, and the joint research project by National Institute for Environmental Studies and local (prefectures and cities level) environmental institutes has worked to standardize the survey methodology based on the guidelines by MOEJ. As part of the joint research project, this manual was prepared to provide a simplified explanations of the survey and key points.