Japan's retail industry, suffering from a decade-long economic slump and the advance of powerful specialty discount stores, is gearing up to compete with another formidable player.

At the end of May, U.S. retail giant Wal-Mart Stores Inc. purchased a 6.1 percent stake in Japan's fourth-largest supermarket chain, Seiyu Ltd., and the new alliance is already accelerating a realignment of the domestic retail industry.

Also in May, Aeon Co., Japan's second-largest supermarket chain, acquired a 26.1 percent stake in midsize supermarket chain Inageya Co. as part of its aggressive strategy to establish a foothold in Tokyo, where Aeon operates just three Jusco stores.