Born in Nara, Japan in 1942, Semmoto graduated from the Department of Electrical Engineering, Faculty of Engineering, Kyoto University. Semmoto later received his Doctor of Engineering degree from the University of Florida, USA. In 1984, Semmoto co-founded Daini-Denden (DDI, now KDDI) with Kazuo Inamori and served as managing director and vice president. In 1996, Semmoto became a professor at Keio University Graduate School and later a visiting professor at the University of California, Berkeley. In 1999, Semmoto founded eAccess Ltd., and served as its representative director. In 2005, Semmoto founded EMOBILE, Inc. (now Y!mobile) and served as its representative director and CEO. In 2014, Semmoto became an outside director of RENOVA, Inc. and went on to serve as Representative Director and Chairman of the Board from 2020.