From downtown Chicago, a former Nomura Holdings Inc. salesman is taking activist investing to Japan's smaller listed firms, betting that Prime Minister Shinzo Abe's corporate governance overhaul is trickling down to less-covered parts of the market.

Masakazu Hosomizu, who works for the ¥890 billion ($8.1 billion) RMB Capital Management, urges small and microcap stocks with big cash holdings to improve how they use capital and run their businesses. If they ignore him, he goes hostile. He's fought several public campaigns over the past two years, with mixed success.

There's nothing strange about Hosomizu's brand of activism, but it's rare for a big U.S. money manager to focus on Japanese stocks that are so small they're barely noticed by analysts. While that's partly due to Hosomizu's unusual past, it's also another sign of how Abe's rewritten rules for companies and investors have encouraged shareholders to work more closely with executives.