In a statement to financial markets, Buffett said he now owns directly or indirectly more than five percent of the voting rights in Munich Re, after taking options on 1.945 percent of its capital in addition to a previous stake of 3.08 percent owned by his
investment fund Berkshire Hathaway.
Shares in the re-insurance group gained 0.23 percent to €110.10 in midday trading at Frankfurt's DAX 30 stock index, which was also up by 0.23 percent.
Buffett also owns 3.2 percent of Swiss Re, a rival re-insurer.
JP Morgan analyst Michael Huttner said Buffett's interest in such groups made sense because they are "really cheap stocks" now. Demand for reinsurance by primary insurers is also forecast to rise, Huttner added.
Re-insurance groups provide coverage for primary insurers while also offering direct insurance themselves and investing in financial markets.