A Japanese court on Wednesday found a martial arts instructor guilty over the death of a six-year-old boy, a court official said, in the first criminal case over judo training in Japan.

The Osaka District Court found the instructor guilty of causing the boy's death by repeatedly slamming him to the floor during training, ordering the defendant to pay a fine of one million yen ($13,000), the official said.

It is the first criminal case filed by Japanese prosecutors against judo trainers, according to a victims' group, despite over 100 child deaths blamed on harsh training or hazing between 1983 and 2010.

The 36-year-old instructor, who owned a private judo club in Osaka, admitted he threw the boy excessively in training. The boy died in November last year from brain swelling, local reports said.

Ryo Uchida, associate professor at Nagoya University Graduate School of Education and Human Development, said at least 114 deaths during judo training had been reported between 1983 and 2010 at schools alone.

"The number of children's deaths, including those outside of schools, like the case of Osaka, remains unknown," Uchida told AFP.

"These serious accidents show that even experienced judo practitioners could give training inappropriately and cause grave injuries or death," he said.

"Instructors must be well aware of the risk of brain injuries and be prepared for emergency treatment."

Keiko Kobayashi, whose youngest child suffered brain damage when he was 15, welcomed the "historic" ruling but questioned if the one million yen fine was sufficient "after one child's life and future was lost".

Judo, which became an official Olympic sport at the 1964 Tokyo Games, has long been seen as a respectable tool for training the minds and bodies of young Japanese and forms a major part of military and police training.

But many argue that abusive trainers are able to escape criminal charges due to the physical risks inherent to the sport.

The All Japan Judo Federation, which recognises 86 judo incidents -- some of them fatal -- in the eight years to 2011, revised safety guidelines in June to warn against the risk of head injuries.