As the world's biggest pension fund nears the end of its switch from sovereign bonds into stocks, investors are looking at Japan Post Bank Co. as the next actor big enough to move markets.

The postal lender, the biggest holder of Japanese government bonds after the Bank of Japan, sold ¥5.1 trillion in JGBs in the three months ended June, after offloading a record amount of the debt last fiscal year. The $1.2 trillion Government Pension Investment Fund, known as "the whale," said last week stock and fixed-income holdings were all within 3 percentage points of their targets, suggesting it has almost completed a planned shift into riskier assets including global bonds and shares.

The BOJ needs to find about ¥45 trillion in JGBs from the market to meet its annual goal for boosting the money supply to stimulate the economy. Japan Post Bank, with 49.2 percent of its ¥206.5 trillion held in domestic debt, fits the profile and needs to seek higher profits ahead of a possible public share sale this year.