Legendary activist investor Paul Singer is taking on one of his most high-profile targets yet: Masayoshi Son.

Singer’s Elliott Management Corp. has built a stake of close to $3 billion in SoftBank Group Corp., according to people familiar with the matter.

Elliott’s substantial investment in SoftBank reflects its conviction that the market significantly undervalues SoftBank’s portfolio of assets, a spokesman for the investment firm said in a statement, declining to comment on the exact size of the stake.

Elliott has met with SoftBank management and is working with the company on suggestions to help increase its shareholder value, according to the statement.

Softbank shares (SFTBY) rose 3.3% at 2:08 p.m. in New York. The Wall Street Journal first reported Elliott’s stake.

‘SoftBank always maintains constructive discussions with shareholders regarding their views on the company and we are in complete agreement that our shares are deeply undervalued by public investor,” a representative for SoftBank said, adding that the company welcomes feedback from fellow shareholders.