Was elected to the Lower House of Diet in April 1946, but soon after lost his seat due to the purge of Japanese officials by the US occupation authorities.
In April 1955 ran against Shinzo Hamai, and won the election after making allegations of financial misconduct by his opponent.
As mayor of Hiroshima, Watanabe supported the notion of establishing nuclear power plants in his city.
After losing the election in 1959, Watanabe returned to his work as a lawyer.
He died of uremia at Hiroshima Municipal Hospital at 2:35 p.m. (Japan time) on May 6, 1980