Japan Post Insurance's holdings of Japanese government bonds (JGB) fell to about 49trn yen ($411bn) at the end of December, down from 52.5trn yen at the end of March, according to Reuters.
During the same period, foreign securities held by the company rose 62% to 2trn yen. Japan Post Bank also announced a similar change. Both companies are subsidiaries of Japan Post Holdings.
The reason for the decline was the recent fall in JGB yields, which accelerated after the Bank of Japan (BOJ) launched a surprise additional stimulus in October. The benchmark 10-year JGB yield, which had been above 0.4% in October, fell to around 0.35% in December.
"With long-term yields this low, we had no choice but to reduce JGBs," Noboru Ichikura, Japan Post Holdings managing executive, told an earnings briefing.
Ichikura said most of the foreign securities were sovereign and corporate bonds, adding some were bought without being hedged against currency risks.
According to Reuters, the postal giant's two units are among the biggest JGB holders and its investment strategy has been watched closely since the BOJ launched a monetary easing programme in 2013.
The country's top banks have been actively reducing their JGB holdings and life insurance companies have started looking for higher-yielding assets.
In May 2014, Japan Post Insurance announced that it was investing $3.5bn in Japanese stocks and foreign bonds in pursuit of higher returns (IAR, 22 May 2014, Japan Post invests $3.5bn in Japanese assets).