Meiji Yasuda Life Insurance Co. said Friday it will buy U.S. insurer StanCorp Financial Group Inc. for about ¥624.6 billion ($5 billion) in what will be one of the biggest takeovers of an overseas company by a Japanese life insurer.

Tokyo-based Meiji Yasuda, the third-largest life insurer in Japan by premium income, aims to expand its business abroad in the face of little room for growth in the fast-graying domestic market.

"This continues the process" of Japanese insurers pursuing takeovers abroad, said David Threadgold, an analyst at Keefe Bruyette & Woods in Tokyo. Japan's demographics mean that "if you stay at home, you're a shrinking company."