Matsuda condition 'critical' after collapse
The 34-year-old, who played for Japan at the 2002 World Cup, went into cardio-respiratory arrest, according to his JFL team Matsumoto Yamaga.
Medics rushed to Matsuda's aid after he collapsed some 35 minutes after training began at 0930 local time.
Yamaga president Hiroshi Otsuki told a news conference Matsuda had been put on an artificial respirator to keep his blood circulating, adding that the player's condition remained "very bad."
Doctors suspect the player may have suffered a heart attack after he arrived at hospital unconscious and not having responded to heart massage, his club said.
Matsuda has played 40 times for Japan and made 385 appearances for Yokohama F-Marinos, helping them win three J-League titles before moving down to the JFL this year.