Sompo Japan Nipponkoa Holdings Inc. will buy a 15 percent stake in French re-insurer Scor SE in potentially its biggest-ever overseas investment.

The insurer will initially buy 7.8 percent from Swiss fund Patinex AG and eventually increase ownership to at least 15 percent, including by purchasing the stock from the market, Tokyo-based Sompo Japan said Friday.

While Sompo Japan didn't give a price, a 15 percent stake would be valued at about €860 million (¥113 billion), according to data compiled by Bloomberg based on Thursday's close. That would top Sompo Japan's £594 million (¥108 billion) purchase of U.K. insurer Cans Group Ltd. last year.

Japan's fourth-biggest publicly traded insurer is expanding in Europe and the Americas to obtain stable profit contributions, it said. The stake in Scor may boost Sompo Japan's annual net income by about ¥10 billion, a source with knowledge of the matter said earlier, asking not to be identified before the announcement.