Nishimura & Asahi, which helped Mitsubishi Estate Co. convert Towa Real Estate Development Co. into a wholly owned subsidiary, was the nation's top-ranked legal adviser on mergers and acquisitions last quarter.

Tokyo-based Nishimura & Asahi, the nation's biggest law firm, topped the rankings by both value and number of deals for the first time, data compiled by Bloomberg showed. The firm, which also helped advise USJ Co. on Goldman Sachs Group Inc.'s takeover bid for the theme park operator, was involved in 27 deals worth a total of $4 billion. Nishimura & Asahi ranked third in terms of the number of deals and seventh in terms of deal value a year earlier.

Foreign law firms, which have an edge in cross-border deals, had previously dominated top spots by deal value, while Japanese firms led the list by the number of deals. No firms had taken top places in both rankings since the first quarter of last year when Bloomberg started compiling the data.

The aggregate value of mergers and acquisitions involving Japanese companies dropped 28 percent to $25.2 billion in the three months that ended March 31 from a year earlier.

"Companies based in Japan are increasingly hiring domestic law firms in cross-border deals," said Naohisa Fukutani, a partner at GCA Savvian Corp., a Japanese independent merger and acquisition adviser. "Efforts by Japan's top three firms to aggressively recruit lawyers to meet demand have paid off."

In terms of the number of deals, Tokyo-based Nagashima Ohno & Tsunematsu was second with 17 deals, followed by Mori, Hamada & Matsumoto with 15 deals.