Was skeptical about this data.

Our World In Data sites a 2015 study [1] which in turn cites an 2012 World Bank report [2] for some of its data, including the numbers for Japan. This then in turn cites OECD data without a solid link, but a search and some digging leads us to [3]. This data is acquired through questionnaire (there's some info available in the data explorer), so the trail runs cold there.

I did cross-check with the numbers of the Central Bureau of Statistics of the Netherlands, and the overall waste production matches: an average 532 kg of waste per capita, ~17 million people leads to about 9000 million tons of waste per year.

However, I cannot find the fraction of plastic waste on the OECD site anywhere. If I use the Dutch CBS data (counting "Kunststof verpakkingen", "PMD-fractie", and "Harde plastics"), the Dutch fraction of municipal waste that is plastic-related seems to be only about 4%, which is 5 times fewer than the World Bank report lists.

[1] https://www.science.org/doi/10.1126/science.1260352

[2] https://openknowledge.worldbank.org/entities/publication/1a4...

[3] https://data-explorer.oecd.org/vis?fs[0]=Topic%2C1%7CEnviron...

[4] https://www.cbs.nl/nl-nl/cijfers/detail/83452NED?dl=41FD6