TOKYO (Reuters) - Japanese telecoms conglomerate SoftBank Group on Monday announced its biggest ever buyback in a bid to boost investor confidence, saying it will purchase up to 500 billion yen ($4.4 billion) worth or as much as 14.2% of its own shares.

The stock has fallen over 37% year-on-year amid worries about SoftBank's outlook, including the future of loss-making U.S. wireless carrier Sprint Corp, in which it has a majority stake.

The move follows its buyback of around $1 billion worth of shares in August, which Chief Executive Officer Masayoshi Son had said was partly in response to renewed confidence in Sprint.

It also comes after SoftBank last week reported a better-than-expected 7.3% rise in third-quarter operating profit, as Sprint trimmed losses on subscriber gains and cost cuts.

Naoki Yokota, analyst at SMBC Friend Research Center, said investors will respond favorably to the buyback, which SoftBank has said will be carried out over a year starting on Wednesday.

"SoftBank shares have become so cheap now. For the company to say it's buying back at this time will have an 'announcement effect'," he said, while adding that a full-fledged recovery still depended on a turnaround at Sprint.

(Reporting by Makiko Yamazaki and Ritsuko Ando; Editing by Edwina Gibbs and Miral Fahmy)