NTT DoCoMo Inc. plans to experiment from next March with a service that will allow mobile-phone users to transfer money from their bank accounts to plastic cards used for electronic commerce, company officials said Wednesday.

NTT DoCoMo, Japan’s largest mobile-phone group, is currently discussing the plan with partners Sony Corp. and NTT Data Corp. as well as with local governments, the officials said.

NTT DoCoMo is considering using 300 mobile-phone users in Sapporo for the trial, they said.

Currently, users of electronic-money cards typically have to “refill” them by accessing the Internet via personal computers, which can be inconvenient if the cards run out of money on shopping trips.

The planned service would allow the cards to be topped up via mobile phones, the officials said.

Users would transfer money from their bank accounts to their mobile phones or other wireless data terminals, and then to their cards via dedicated devices, the officials said.

The experiment would be conducted at participating shops and convenience stores in a designated area, they said.

The cards could also be used when paying subway and bus fares as well as buying beverages from vending machines, they added.

30 million use DoCoMo

The number of subscribers to mobile-phone services operated by NTT DoCoMo Inc. exceeded 30 million for the first time in May, capturing 57.5 percent of the market, according to data released by mobile-phone operators by Wednesday.

The number of NTT DoCoMo subscribers rose 393,000 in May over the previous month to 30.391 million.

The Cellular Telephone group of eight operators belonging to DDI Corp. had 6.2834 million subscribers in May, down 3,600, due to a decrease of 170,700 subscribers to the conventional digital mobile-phone service.