Japanese investors are scooping up bonds overseas to flee negative yields at home, driving a global debt-market rally.

Money managers purchased a net ¥4.45 trillion of debt abroad from Feb. 12 to March 4, the biggest three-week outflow since the third quarter of 2010, based on Finance Ministry data. Nippon Life Insurance Co., the nation's biggest life insurer, called it an "unavoidable" trade.

The shift to foreign bonds has accelerated after the central bank announced plans at the end of January to cut the benchmark interest rate to below zero, pushing 10-year yields in the nation to as low as minus 0.1 percent this week. Ten-year notes yield 1.89 percent in the U.S. and 0.63 percent in France.

"People have to move out of their deposits, or from zero percent rates in the money market," said Hideo Shimomura, the chief fund investor at Mitsubishi UFJ Kokusai Asset Management in Tokyo, which oversees about $106 billion. "They're looking for coupon. They're not going to equities or riskier assets." Treasuries as well as French, Italian and Spanish bonds are in demand, he said.

The Life Insurance Association of Japan said in February that negative interest rates make it difficult for the nation's insurers to manage their assets and products.

"It's unavoidable that we will see a shift to foreign debt," Yoshinobu Tsutsui, the chairman of the association and Nippon Life's president, said at the time. Ensuring the payments on insurance policies "has been our mission, and at the current level of interest rates, it would be difficult to manage assets that focus on Japanese government bonds."

Global demand for debt has pushed the yield on Bloomberg Global Developed Sovereign Bond Index down to 0.76 percent from 2015's high of 1.24 percent set in June. The yield dropped to 0.69 percent in February, the lowest ever for the index, which goes back to 2010.