  This is a brief summary of an introductory lecture for students and scholars
in general given by the author at Nambu Memorial Symposium which was held at
Osaka City University on September 29, 2015. We review the invention of string
theory by Professor Yoichiro Nambu following the discovery of the Veneziano
amplitude. We also discuss Professor Nambu's proposal on string theory in the
Schild gauge in 1976 which is related to the matrix model of Yang-Mills type.
