Following studies in Europe and the United States, he was promoted to full professor of commercial law at the university in 1923.

In the final months of World War II, Tanaka joined a group of Japanese intellectuals that sought peace with the Allies in opposition to Japan's militarist government.

He was the first Christian to hold this position, and drew upon his international legal training to interpret the new Japanese Constitution.

This friendship led to controversy after Tanaka presided over the Supreme Court's December 1959 decision in the "Sunagawa Case," which ruled that Article 9 of the Japanese Constitution did not preclude Japan from taking certain defensive measures, and enabled the ongoing United States military presence in Japan by refusing to clearly declare it constitutional or unconstitutional.

[4] Tanaka wrote the 1957 Supreme Court decision that banned the book Lady Chatterley's Lover in Japan on the basis that it offended public standards of decency.

Tanaka concluded that UN member states had "the legal obligation to respect human rights and fundamental freedoms."