TOKYO, Dec 6 (Reuters) - Japan's Rakuten Group said on Wednesday it plans to sell a large stake in Rakuten Bank to overseas investors, its latest fundraising effort as it grapples with heavy debt and losses at its mobile network unit.

The planned sale of 25.5 million shares is equivalent to just under 15% of the online bank, according to LSEG data, and is worth around 70 billion yen ($475 million) at Wednesday's closing price.

The e-commerce and financial services group said it would use the proceeds to repay bonds early as it was committed to reducing interest-bearing debt. Hit by spiralling costs to build out its mobile network, it has almost 800 billion yen ($5.4 billion) in bond redemptions due before the end of 2025.

The unit's troubles have also resulted in Rakuten logging 13 consecutive quarters of operating losses.

Since 2021, Rakuten has issued new shares to strategic investors and the public, twice sold down its holding in its securities arm Rakuten Securities, and listed Rakuten Bank in April of this year.

The listing raised 72 billion yen and reduced Rakuten's holding in the bank to 63.3%.

The price of the shares will be determined after a book building period between Dec. 6 and 7, Rakuten Bank said in a separate statement.

Analysts predict Rakuten may soon seek to list its credit card business Rakuten Card, which includes the group's lucrative points and payments system.

($1 = 147.04 yen) (Reporting by Anton Bridge and Mariko Katsumura; Editing by Miral Fahmy and Edwina Gibbs)