The Bank of Japan held more than 581 trillion yen in government bonds as of the end of last month, the highest ever at the end of the fiscal year. The main reason is that the Bank has continued to purchase government bonds as part of its large-scale monetary easing to curb the rise in long-term interest rates.
Source
Government bonds held by the Bank of Japan exceed 581 trillion yen, the largest ever with unlimited purchases | NHK
