The government of Japan released The Status Report of Plutonium Management in Japan - 2019, which details its plutonium holdings. According to the report,
As of the end of 2019, the total amount of separated plutonium both managed within and outside of Japan was approximately 45.5 tons, approximately 8.9 tons of which was held domestically and the rest of approximately 36.6 tons was held abroad.
The amount of domestic storage was approximately 8.9 tons at the end of 2019, as electric utilities (Kyushu Genkai unit 3) irradiated approximately 0.2 tons of separated plutonium.
Of the plutonium stored abroad, 15,435 kg are stored in France (15,460 in 2018) and 21,180 kg - in the United Kingdom (21,205 in 2018). The reprocessing of Japan's spent fuel held in France had been completed by the end of 2017. The United Kingdom still holds about 0.6 tonnes of Japan's plutonium in spent fuel. It appears that none of that material was separated since the end of 2017.
In 2018, Japan reported having a total of 45.7 tons of separated plutonium, of which 9.0 tons were held domestically.