The official line appears to be that JGB are (relatively) risk-free assets: foreign bonds are “riskiER”.

TOKYO (Nikkei)–The nation’s largest life insurers continue to invest most of their assets in Japanese government bonds despite efforts by the Bank of Japan to get them to buy riskier assets, such as stocks and foreign bonds.

via 2013/08/10 05:18 – Life Insurers Make Little Headway In Shift To Foreign Bonds.