Club officials say former Japan defender Naoki Matsuda collapsed during training after suffering a cardiopulmonary arrest.
The 34-year-old Matsuda was unconscious when he arrived at a hospital Tuesday and has been on an artificial cardiopulmonary unit, Matsumoto Yamaga Football Club team officials say.
His condition was "extremely severe," club president Hiroshi Otsuki said, according to doctors.
Matsuda collapsed after finishing a 15-minute warmup run during training, coach Yoshiyuki Kato said.
Matsuda played 40 times for Japan including at the 2002 World Cup held in Japan and South Korea.
Record high temperatures across Japan have caused a rise in heatstroke cases. A total of 43 people have died from heatstroke in the two months through the end of July, Kyodo reported.