TOKYO — When Masayoshi Son, SoftBank’s chief executive, first visited the Silicon Valley fuel cell start-up Bloom Energy late last year, one word came to his mind: crazy.

But the fuel cell technology — which promised efficient, cleaner and increasingly inexpensive “energy in a box” — intrigued him. After several more visits, Mr. Son was convinced that Bloom’s sleek fuel cells were a perfect fit for Japan, energy-poor and made even more so by an almost complete shutdown of its nuclear energy program after the 2011 Fukushima disaster.

Now, SoftBank, the $70 billion Japanese technology investment company that completed its $21.6 billion acquisition of Sprint Nextel this month, is setting up a joint venture with Bloom to bring what it calls “energy servers” to Japan, helping the start-up break into its first overseas market.

The deal adds yet another piece to SoftBank’s fast-growing Internet, mobile communications and technology empire, which increasingly straddles Japan and the United States. It also adds fuel cell technology to SoftBank’s alternative energy portfolio, which already includes solar and wind projects, bolstering its position as an emerging player in electricity generation in Japan.