The ministry's move comes as no small number of smartphone users have signed up for large-data contracts even though their actual data usage falls below the contracted volume.

The ministry aims to prevent overpayment by requesting the carriers to offer customers billing plans that suit their data usage.

According to a research by the ministry, about a half of the three major companies' smartphone subscribers use less than 2 gigabytes of data a month. Only 20 percent of smartphone users, however, sign up for up to 2GB and contracts for about 7GB are the mainstream choice among users.

More than 50 percent of smartphone users said they do not check whether changing to other contracts would save money.

An official of the ministry's Telecommunications Bureau said that "there is a big gap" between the contracted data amount and the actual data usage.

"The challenge is to get users to choose contracts that are right for them," the official added, showing an intention to urge the three mobile companies to fix the problem.

With the ministry's move in mind, KDDI Corp. , one of the three major mobile phone carriers, started Friday a new billing plan under which monthly fees change depending on the amounts of data used.

In order to prevent its customers from switching to lower-priced carriers, the new plan will enable subscribers to use their smartphones at a monthly fee as low as 1,980 yen before tax for the first year.

Industry rivals NTT Docomo Inc. and Softbank Group Corp.  have not yet taken similar measures. (Jiji Press)