TOKYO—Japan's Meiji Yasuda Life Insurance Co. said it agreed to buy StanCorp Financial Group Inc. for $5 billion, the latest in a series of acquisitions by Japanese firms abroad.

The purchase would be one of the largest for a Japanese insurance company. In June, Tokio Marine Holdings Inc. agreed to buy U.S.-based HCC Insurance Holdings Inc. for $7.5 billion.

Meiji Yasuda rival Dai-ichi Life Insurance Co. closed a deal earlier this year to acquire U.S. insurer Protective Life Corp. for $5.6 billion.

The board of Portland, Ore.-based StanCorp unanimously supported the deal with Meiji Yasuda, the Japanese life insurer said in a statement. The deal is still subject to regulatory approvals in the U.S. and Japan.

Write to Kosaku Narioka at kosaku.narioka@wsj.com

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