Billionaire investor Warren Buffett’s Berkshire Hathaway (BRKA) has added to its holdings in Japan’s five biggest trading houses, likely underpinning strong momentum propelling the nation’s stock market to multi-year highs.

Berkshire said Monday its stakes in Itochu (ITOCF), Marubeni (MARUY), Mitsubishi Corp, Mitsui & Co (MITSY), and Sumitomo now average more than 8.5%.

It first announced the buys in 2020, and the additional purchases are in line with its plans to hold the stakes long-term and increase them to as much as 9.9%.

Buffett’s investments and his optimism about Japan’s prospects have drawn attention to the country’s improving economic conditions and shareholder-friendly corporate governance reforms that have helped underpin a sparkling rally in the Nikkei (N225)share average.

The market ended 1% lower Friday, and Berkshire’s announcement came after Monday’s close, but 10 weeks of consecutive gains have helped the Nikkei rise 28% this year.

Berkshire’s biggest bet Outside America

Berkshire said the aggregate value of the investments is the largest of any Berkshire-held public stocks outside the United States.

Known as “sogo shosha,” Japanese trading houses deal in a variety of materials, products, and food, often serving as intermediaries, and providing logistical support.

The stocks are all up more than 30% this year, with Marubeni shares up 62%. That stock has more than tripled in price since the end of 2020.

The trading firms’ regulatory filings of June 12 showed Berkshire holding 7.4% of Itochu’s stock, 8.3% of Marubeni and Mitsubishi’s stock, 8.1% of Mitsui’s stock and 8.2% of Sumitomo’s stock.