A Japanese life insurer has invested about $400 million in the stock of a U.S. life insurer this week.

Nippon Life Insurance Company, Tokyo, moved Tuesday to increase its stake in Principal Financial Group Inc., Des Moines, Iowa, to 7% of Principal’s total common stock, from 4.3%, according to Principal.

Nippon Life is now Principal’s biggest single investor.

“Over the course of a long-term business relationship, we’ve built a strong history of cooperation and collaboration with the Principal,” Kiyoshi Ujihara, managing director of Nippon Life, says in a statement about the investment. “We are very pleased to affirm the importance of our existing relationship and to demonstrate our continued support by increasing our ownership position.”

Larry Zimpleman, president of Principal, says Principal deeply values its relationship with Nippon Life.

“We truly appreciate the confidence Nippon Life has shown in us through their increased ownership stake, and will continue to execute our growth strategy as an independent company to deliver strong shareholder returns,” Zimpleman says in a statement.