All-Japan Judo Championships
Sekine was born in Ōarai, Ibaraki, and entered the Tokyo Metropolitan Police Department after graduating from Chuo University. He sought a spot on the Olympic judo team after seeing Isao Okano, a rival judoka also from Ibaraki Prefecture, win gold at the 1964 Summer Olympics. However, judo was not included in the program for the 1968 Summer Olympics, and Sekine entered the Olympics for the first time in 1972 as a 28-year-old veteran after winning the All-Japan Judo Championships that year. Sekine lost to Oh Seung-Lip of South Korea in the 5th round of the tournament, but won the repechage to face Oh for the second time in the Olympic final. Sekine was forced to fight defensively for most of the match, but in the few remaining seconds, he tried a Tai Otoshi which put his opponent down onto the mat. The two assistant referees were split on the outcome, but the main referee from the Netherlands ruled in favor of Sekine to award him an extremely close decision win. Oh had been leading in points for most of the match. Sekine retired shortly after winning the Olympic gold medal, and served as a coach and advisor for the All-Japan Judo Federation, and as a referee during the 1996 Summer Olympics. He also worked as an instructor for the Tokyo Police Department, and Heisei International University.