The mobile phone market is largely saturated, however, and fees are being driven down by an ongoing price war. For Softbank, a government-backed health-monitoring service could be a boon to business.

GPS technology has its shortcomings, including hazy readings indoors. But Softbank believes it could keep readings accurate to several yards, at least for an experiment in a limited area.

Until now, technologies such as GPS have mainly been used to help people figure out where they are and what is nearby. As networked devices like the iPhone become more popular, new applications let people track their children or friends, and they could give companies and governments access to their location.

Aoyama Gakuin University, a prestigious school in Tokyo, is giving Apple Inc.’s iPhone 3G to students, partially as a way to check attendance via GPS readings from an application running on the phone.

That kind of project raises privacy concerns, and one of the goals of the Japanese experiment is to judge how participants feel about having their location constantly recorded.

If a disease-tracking system were launched for real, no one would be required to sign up, said Takuo Imagawa, an official at the Ministry of Internal Affairs and Communications.

Another concern for the experiment is how to inform people that they might be infected, even if it’s just a virtual disease.

“If we don’t think carefully about the nature of the warning, people that get such a message could panic,” said Katsuya Uchida, a professor at the Institute of Information Security in Yokohama. Uchida serves on a board that evaluates such proposals for the government.

Softbank Telecom, the subsidiary that made the original proposal, might not be chosen by the ministry to run the experiment in the fall. But Takahashi says whichever company is chosen, he hopes the potential benefits of a monitoring system are enough to persuade people to sign up and reveal their whereabouts.

“I think it would have a bigger impact than Tamiflu,” he said, referring to a leading prescription drug for treating flu symptoms.