By Carlo Versano

SoftBank Group, the Japanese multinational conglomerate with stakes in some of the hottest U.S. tech start-ups, is planning a massive IPO for its telecom arm that could become Japan's largest public offering to date, Bloomberg reported Monday.

SoftBank Founder Masayoshi Son is reportedly seeking $21 billion for the new entity that will be called SoftBank Corp. and begin trading Dec. 19. Citing the prospectus, Bloomberg said the company will offer 1.6 billion shares at 1,500 yen a piece, or roughly $13. That figure could change as the range of the offering is set.

Son has used his $100 billion Vision Fund to transform SoftBank into a holding company with stakes in some of the world's most valuable technology companies, including Uber, WeWork, and Alibaba ($BABA). Softbank's telecom and mobile operation has long been the company's bread and butter, and spinning it off will give Son room to further position himself as a tech kingmaker.