NTT DoCoMo Inc., the nation's biggest mobile-phone carrier by subscribers, aims to keep boosting operating profit by curbing overall spending as two smaller rivals add more customers by offering Apple Inc.'s iPhone.

The company plans to maintain annual spending at about ¥700 billion in the four years starting next fiscal year, Chief Financial Officer Kazuto Tsubouchi said in an interview Friday. More of that will be spent building faster networks to compete with KDDI Corp. and Softbank Corp., he said.

"Our coverage is already 100 percent in the third-generation network," Tsubouchi said. "We plan to spend more on the long-term evolution network, as it can build on the 3G network."

DoCoMo, which posted gains in operating profit the past four years, is trying to capitalize on the faster Xi data-communication service it started offering last December. It raised its budget for expanding Xi by 10 percent to ¥330 billion for the three years ending in March 2013, it said Nov. 2.

KDDI and Softbank plan to offer the service next year.

DoCoMo, which doesn't offer the iPhone 4S, lagged behind in adding customers in October after Softbank and KDDI began selling the latest Apple handset.

Softbank added 247,600 new customers in October, followed by KDDI's 196,900, according to figures released Nov. 8. DoCoMo's new users totaled 89,600 last month, the company said.