The positive test is the first ever by a Japanese athlete at the Winter Olympics.
"I've never used body-enhancing drugs so I don't think about hiding it".
Japan's chef de mission Yasuo Saito said that Kei Saito had a doping test at a 29 January training camp for the Games and had tested negative.
This is the first doping violation case registered by the CAS Anti-doping Division in PyeongChang. "The only way this substance could have entered my system is that I took it unknowingly and happened by accident".
Saito, a human biology student, also participated in the Winter Youth Olympic Games in 2012, when he placed fifth in the 500m, fourth in 1,000m and seventh in Relay X. So I have no motivation to use this.
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Saito has tested positive for acetazolamide, a diuretic, in an out-of-competition check, and "accepted on a voluntary basis to be provisionally suspended and to leave the Olympic Village", according to a CAS statement. "It is impossible for us to submit any evidence that prove them otherwise during the limited time", Japan's delegation leader said.
The most notable case of Japanese athletes violating anti-doping rules came at the 1984 Summer Olympics in Los Angeles, where members of the Japanese men's volleyball team tested positive for a stimulant.
Japan's short track speed skating head coach Tsotomu Kawasaki added: "I feel the same way as the athlete: I am surprised and dismayed, this is all I can say about this". Last month, however, sprint canoeist Yasuhiro Suzuki admitted to spiking a rival's drink with an anabolic steroid to scuttle his Olympic dream, and was banned for eight years.
The 21-year-old released a statement in which he denied intentionally using a banned substance and saying that he was "extremely shocked" by the positive results.