SoftBank Group Corp. is prepared to spend a record ¥500 billion buying back stock after the carrier saw its shares drop to their lowest since buying Sprint Corp. in 2013.

SoftBank will purchase as many as 167 million shares, or 14.2 percent of its stock, using cash holdings and the proceeds of asset sales, according to the Tokyo-based company. The company, saddled with about $100 billion of total debt, said it will not resort to more loans to fund the program.

Chairman Masayoshi Son has been dogged by doubts about his ability to turn around Sprint as investor concerns drove a 28 percent plunge in SoftBank stock so far this year. That steep decline has pushed the Japanese company's market value below that of its own investments in companies including Alibaba Group Holding Ltd.