One assumption, 36.6 trillion yen
For the year beginning next April, the finance ministry in Tokyo has asked for about 36.6 trillion yen to service Japan's debt, roughly 5.4 trillion yen more than the current year's initial budget. About 16.6 trillion yen of that is interest payments alone, a line running 27.2 per cent above this year's. None of those figures describes a new borrowing decision; they describe the price of borrowing already done.[1]
The mechanical driver is a single line in the request: the assumed interest rate on long-term government bonds rises to 3.8 per cent for the coming year; for the current year it stood at 3.0 per cent. That figure reflects the planning convention the ministry uses to build the budget. The same convention turns a stock of past borrowing into a cash cost the budget has to carry. Long-term yields have climbed alongside inflation and doubts about the fiscal position.[1]
Where the burden actually arrives
Sovereign debt reprices at the speed of its own maturities. Each bond that comes due is replaced at whatever coupon the market asks that week, so a stock accumulated at near-zero yields converts into a higher cash bill gradually, in the order the calendar sets. That is how the interest line can jump 27.2 per cent while the borrowing decisions behind it are years old. The competing reading deserves a hearing: ministries habitually set the assumed rate cautiously, and the eventual outturn may come in below the request.[1]
The rate path behind that assumption is already being priced. At the Bank of Japan's September 18 decision, traders were assigning roughly an 80 per cent probability to an increase. A former Policy Board member expects the bank to move then and again as early as January, warning that leaving settings unchanged could reignite a sell-off in the yen.[2]
Buffers, and the threshold to watch
Several things still absorb the shock. A budget request is an opening bid, and the debt-service line is fixed only when the initial budget is set. The interest figure covers a year that has not begun, and a cautiously high assumed rate builds slack into the request instead of draining cash today. The liability runs in yen throughout, so the pressure shows up in the composition of the budget more than in an external funding market.[1]
That leaves a clean threshold. If the Bank of Japan raises in September and again by January, the 3.8 per cent assumption will be tested where it counts: in the debt-service line of the initial budget for the year beginning next April. The signal to watch by 30 April 2027 is simple — whether that line still carries an assumed rate of 3.8 per cent, or a higher one and a larger yen total.[1], [2]