To stop income inequality from growing, Japan should levy a heavier tax on big earners, said Thomas Piketty, a French economist known for his recent best-seller "Capital in the Twenty-First Century."

The amount of national income held by a small number of wealthy people in Japan has climbed in the past few decades amid the prolonged economic slump, although not as sharply as in the United States, he said in a symposium in Tokyo on the widening income gap and Japan's future.

"Inequality in Japan is in some ways closer to Europe than to the U.S.," he said Thursday.