The Bank of Japan posted a net profit of ¥628.8 billion ($5.13 billion) in the April-September first half, thanks mainly to interest from its massive purchases of Japanese government bonds, it said.
The profit, up 7 percent from a year ago, marks the central bank's highest half-year figure since 2001.
The BOJ started gobbling up state debt under the radical quantitative and qualitative ultra-easy monetary policy it launched in April 2013 and expanded in October 2014, in a bid to lift the world's third-largest economy out of nearly two decades of deflation.
The central bank's total assets have thus ballooned 32.1 percent to a record ¥366.13 trillion, with JGBs accounting for ¥309.57 trillion of it, up 35 percent.
Interest income from the JGB holdings meanwhile surged 23.8 percent to ¥639.1 billion, the BOJ said.
But the bank's appraisal gain on its foreign exchange holdings narrowed sharply to ¥27.2 billion from ¥305.5 billion in the same period last year, because the yen did not weaken as much in the period.
The central bank's capital adequacy ratio stood at 7.94 percent as of the end of September, down from 8.2 percent at the end of the previous March.
The BOJ considers 8 percent a sound level.