The source declined to be identified because the talks are private.

Japan's mobile carrier market is currently dominated by NTT Docomo Inc, KDDI Corp and SoftBank Group .

The Nikkei business daily, which reported on the plan on Thursday, said Rakuten would raise 600 billion yen ($5.3 billion) by 2025 to invest in base stations and other infrastructure.

Rakuten said in a statement that while it was true it is weighing entry into the mobile carrier market, media reports on the matter were not something announced by the company.

Rakuten shares were down 1.7 percent in early trade. The benchmark Nikkei average was flat.