Kotaro Tanaka graduated from Tokyo University (1915) with a degree in German law, and thereafter took a postgraduate course in commercial law.
Kotaro Tanaka was an assistant professor at his alma mater. He made a tour of Europe (1919) and upon his return (1922) rejoined Tokyo University as a professor. mThe same year he earned a doctorate in law for his thesis "Theory of International Law." He was sent to France on an exchange fellowship sponsored by the two governments (1935).
After the end of World War II he was appointed Education Minister in the Shidehara Cabinet (1946). He was appointed chief of the Supreme Court (1950).
He has written several books on law. His "Theory of International Law" was awarded the Asahi Prize (1935).