The government of Japan released The Status Report of Plutonium Management in Japan - 2018, which details its plutonium holdings. According to the report,
As of the end of 2018, the total amount of separated plutonium both managed within and outside of Japan was approximately 45.7 tons, approximately 9.0 tons of which was held domestically and the rest of approximately 36.7 tons was held abroad.
The amount of domestic storage was approximately 9.0 tons at the end of 2018, as electric utilities (Kansai Takahama unit 3, 4 and Kyushu Genkai unit 3) irradiated approximately 1.5 tons of separated plutonium.
Of the plutonium stored abroad, 15,460 kg are stored in France (15,486 in 2017) and 21,205 kg - in the United Kingdom (21,232 in 2017). The reprocessing of Japan's spent fuel held in France had been completed by the end of 2017. The United Kingdom holds about 0.6 tonnes of Japan's plutonium in spent fuel. It appears that none of that material was separated in 2018.
In 2017, Japan reported having a total of 47.3 tons of separated plutonium, of which 10.5 tons were held domestically.