Plastic debris presents a serious hazard to marine ecosystems worldwide. In this study, we developed a method to evaluate high-resolution maps of plastic emissions from the land into the sea offshore of Japan without using mismanaged plastic waste. Plastics were divided into microplastics (MicPs) and macroplastics (MacPs), and correlations between the observed MicP concentrations in rivers and basin characteristics, such as the urban area ratio and population density, were used to evaluate nationwide MicP concentration maps. A simple water balance analysis was used to calculate the annual outflow for each 1 km mesh to obtain the final MicP emissions, and the MacP input was evaluated based on the MicP emissions and the ratio of MacP/MicP obtained according to previous studies. Concentration data revealed that the MicP concentrations and basin characteristics were significantly and positively correlated. Water balance analyses demonstrated that our methods performed well for evaluating the annual flow rate, while reducing the computational load. The total plastic input (MicP + MacP) was widely distributed from 210-4776 t/yr and a map showed that plastic emissions were high for densely populated and highly urbanized areas in the Tokyo metropolitan area, as well as other large urban areas, especially Nagoya and Osaka. These results provide important insights that may be used to develop countermeasures against plastic pollution and the methods employed herein can also be used to evaluate plastic emissions in other regions.