The Bank of Japan made more on its bond holdings on paper in the past year than Mark Zuckerberg is worth.

The valuation gain calculated by comparing the book value of the stake to the market value reached a record ¥4.81 trillion ($39 billion) as of March 31, based on data the central bank issued last week. The profit compares with Facebook Inc. founder Zuckerberg's $35 billion net worth, according to the Bloomberg Billionaires Index.

The central bank and other debt holders have benefited from its policy of gobbling up bonds to combat deflation, while the key inflation gauge is stuck at zero. The more BOJ Gov. Haruhiko Kuroda accelerates purchases to bring consumer price increases to his 2 percent target, the harder it will become to offload the holdings without inflicting losses on the bank and the debt market.