On December 25, 1958, when Watanabe was a Rikkyo University student, he debuted for Japan national team against Hong Kong. He played at 1964 Summer Olympics in Tokyo and 1968 Summer Olympics in Mexico City. At 1968 Summer Olympics, he played 5 games and scored 2 goals against Brazil and France. Japan also won Bronze Medal. In 2018, this team was selected Japan Football Hall of Fame. He also played at 1962 and 1966 Asian Games. He played 39 games and scored 19 goals for Japan until 1969.