One of our soldiers was preparing for drills this morning and opened the door of a building on the base, and there he was," an SDF member told NHK.
"When he asked 'are you Yamato?' the boy said, 'Yes'. Then he said he was hungry, so the soldier gave him some water, bread and rice balls."
NHK said he had told rescuers he "walked through the mountains" until he found the shelter.
He was taken to hospital by a medical helicopter. A doctor later said he was in very good condition, despite only having had water during his six-day ordeal.