Rakuten posted a January-March loss of 102.7 billion yen for the mobile business.
Rakuten founder and CEO Hiroshi "Mickey" Mikitani originally outlined plans to becoming Japan's fourth major carrier, promising to create a low-cost nationwide mobile network by using cloud-based software and commoditised hardware.
However, the company has burned cash funding the build-out, with analysts flagging Rakuten's struggle to take market share from cash-rich incumbents known for high-quality networks.
($1 = 135.0500 yen)
(Reporting by Sam Nussey; Editing by Christopher Cushing)