All-Japan Judo Championships
Natsui competed several times at the All-Japan Judo Championships in Nippon Budokan in Tokyo—one of the three major tournaments, together with the Olympic Games and the World Championships. In 1951 he finished 8th, he finished 16th in 1952, and 8th again in 1953. He received a bronze medal in 1954, a silver medal in 1955 (losing to Yoshimatsu in the final), and a gold medal in 1957. The All-Japan Judo Championships where not held in 1956 because of the inaugural World Championship competition.