Japan Post Insurance Co., the nation's biggest insurer by assets, plans to increase investments in foreign debt to boost returns as the central bank's negative interest rate policy sends government bond yields to unprecedented lows.

The insurer, which has more than half of its ¥82.7 trillion of assets in Japanese government bonds, will lift its allocation to stocks and foreign debt to 10 percent of the portfolio by March 2018 from 6.4 percent as of December 2015, Chief Executive Officer Masami Ishii said.

"We will probably put more weight on foreign bonds" and consider increasing hedging against currency risk on such investments given the yen's recent advance, Ishii, 63, said in an interview in Tokyo. "We'll have to figure out ways to maintain a balance between the cost of hedging and investment."