Softbank Corp., which runs the only Japanese mobile operator to offer the iPhone, said profit doubled in the latest quarter as its cellular business continued to expand.

Net profit was ¥43.37 billion during the July-September period, up from ¥21.75 billion a year earlier, the company said Thursday. Revenue was nearly unchanged at ¥682.9 billion versus ¥681.7 billion.

Softbank said profit increased even as sales held steady because of falling costs as its mobile operations became a bigger part of its overall business. Softbank paid less to access the networks of other carriers and its per-unit costs for phones dropped as overall shipments increased.

Softbank, which also runs major Web properties, including Yahoo Japan and a large stock-trading site, said lower online sales also meant lower expenses.

The upstart carrier is the smallest of Japan's big three mobile companies, but has aggressively undercut rivals with cheap calling plans and introduced flashy ad campaigns with major stars. It has also secured exclusive rights to hit products like the iPhone from Apple, which is well-suited to checking the Internet and so boosts income from data charges to customers.