A video showing a volleyball coach repeatedly slapping a schoolboy -- less than a fortnight after Tokyo was awarded the 2020 Olympics -- is the latest example of brutality to tarnish Japanese sport.
A short clip posted on YouTube showed the teacher at Hamamatsu Nittai senior high school in central Japan smacking the student's face at least 13 times in 16 seconds. It was authenticated by the school.
The episode was captured on a mobile phone by another student during a practice game in Gifu, northwest of the city, on Sunday. By lunchtime Wednesday, it had attracted almost 1.5 million viewings.
"Don't joke around, kid! Do you understand? You're stupid," the teacher yells in the video as he repeatedly slaps the child's face.
According to the school, the teacher has admitted the physical abuse of the second-grader, saying: "I wanted to shake him up, but I went about it the wrong way".
Students in Japan are 16 or 17 years old in second grade of high school.
Toshitaka Shiozawa, assistant principal, told AFP the 41-year-old teacher had also beaten another student on the same day. He declined to reveal the teacher's name.
Neither student suffered any lasting injuries in the attacks, Shiozawa said, adding the school was considering punishing the teacher.
Japan banned corporal punishment in schools after World War II, but it remains far from uncommon, particularly in sports education, despite a number of high-profile cases.
In December, a teenager killed himself following repeated physical abuse from his high school basketball coach in Osaka.
Earlier this month, world judo champion Shohei Ono was suspended from his university for physically abusing junior members of the judo squad.
Japan's judo community was rocked in January when it emerged the coach of the national women's team was found to have beaten athletes, sometimes using a bamboo sword, calling his charges "ugly" and telling them to "die" in the run-up to the London Olympics.
The latest incident came less than two weeks after Japan was awarded the right to host the 2020 Olympic Games, and followed the announcement that the government was to create a sports agency to boost elite athletes' performance.