Aioi Insurance Co. said Thursday it will invest about ¥2.8 billion to buy 9.9 percent of Lotte Group's nonlife insurer to strengthen ties with the South Korean firm.

Aioi will acquire 4.16 million outstanding shares in Lotte Non-Life Insurance Co. the Tokyo-based insurer said.

Lotte Non-Life, based in Seoul, is a unit of South Korea's fifth-largest family-run industrial group. Aioi is Japan's fifth-largest casualty insurer by market value.

The purchase will make Aioi the third-largest shareholder of Lotte Non-Life, based on the South Korean firm's latest quarterly financial statement June 30. Aioi said that Lotte Non-Life had about ¥59 billion in premium revenue last fiscal year.