There seems to be no end in sight for the price war among mobile phone carriers, with Rakuten Mobile Inc. unveiling a lower cost plan Friday in an effort to keep its edge over its powerful rivals.

Until now, Rakuten has been offering only one choice on price: unlimited mobile internet for ¥2,980 a month. But the firm now says it will lower the price depending on the amount of data used within the space of a month, saying the fee will even be free for particularly light users.

Starting in April, Rakuten Mobile will charge: