Yoshikazu Tsuno/Agence France-Presse — Getty Images

Softbank, the cellphone service provider that brought Apple’s iPhone to Japan, said on Monday that it would buy a smaller rival, eAccess, for about $2.3 billion in stock, in the face of intensifying competition.

Under the terms of the deal, Softbank will pay 52,000 yen a share, more than three times the eAccess closing price of 15,070 yen on Friday.

The large premium reflects Softbank’s desire to bolster its mobile broadband capacity as it looks to attract more smartphone users. After the deal, Softbank would have more than 39 million subscribers, making it the second-biggest network operator in Japan, behind NTT DoCoMo.

“We do not think our shareholders will view this as a bad tie-up,” Softbank’s chief executive, Masayoshi Son, said in a statement. “I never liked third place. We fought for No. 2, and someday, we will be No. 1.”

Shares in eAccess rose 26 percent on Monday, to 19,000 yen, while those in Softbank slumped 1.7 percent, to 3,105 yen.

The deal underscores Japan’s changing mobile phone industry.

Softbank brought the iPhone to Japan, winning an exclusive contract with Apple in what some analysts described as a coup. While Softbank trailed KDDI by over 10 million subscribers at the time, Softbank has rapidly expanded its user base on the back of the explosive popularity of the iPhone.

But Softbank is now dealing with a more crowded field. Last year, KDDI began offering the popular Apple smartphone as well.

Since then, Softbank, whose networks are under pressure from the surge in smartphone users, has experienced a steady trickle of customers switching to KDDI, where users reported faster network speeds.

With the eAccess acquisition, Softbank will extend its Long Term Evolution services, a next-generation high-speed wireless network that is compatible with the iPhone 5. The deal will give Softbank 50 percent more LTE base stations, increasing them to an expected 30,000 by March.

“There is a severe battle going on,” Mr. Son said. “It’s all about who can expand the network fastest.”