On August 13, 1971, when Nagai was 19 years old, he debuted for Japan national team against Iceland. He also played at 1972 Summer Olympics qualification in 1971. Although, he did not play for Japan in 1972, he was selected Japan for 1974 World Cup qualification in 1973. He also played at 1974, 1978 Asian Games. In 1980, he was selected Japan for 1980 Summer Olympics qualification, Japan's failure to qualify for 1980 Summer Olympics. This qualification was his last game for Japan. He played 69 games and scored 9 goals for Japan until 1980.