(Reuters) – Billionaire investor Warren Buffett’s Berkshire Hathaway (NYSE:BRKa) said it added to stakes in Japan’s five biggest trading houses to beyond 8% on Monday, a move likely to highlight and add to the momentum driving Japan’s stock market to new heights.

Berkshire announced stakes in Itochu, Marubeni, Mitsubishi Corp, Mitsui & Co and Sumitomo in 2020 and adding in line with plans to hold the stakes long term and increase them as far as 9.9%.

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

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

Berkshire said its ownership now averages more than 8.5% at the companies and the aggregate value of the investments is the largest of any Berkshire-held public stocks outside the U.S.

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

The stocks are all up more than 30% this year, with Marubeni shares up 62% and having more than tripled in price since the end of 2020. Nikkei futures slightly pared some losses after the Berkshire announcement.