Yamato's parents had briefly left him by a wooded road near Nanae in Hokkaido region to punish him for throwing rocks on a family day out. When they went back minutes later he had gone.
He was dressed in only a T-shirt and jeans, in an area where temperatures can dip as low as 9C at night.
"It's been colder than it usually is at this time of year," Ross Findlay, founder of the Niseko Adventure Centre in the region, told the BBC.
"We've had snow on top of the mountain in Niseko, though that might not have been the case of Nanae, which is further south. But for a small boy who doesn't have much body fat, that's a problem." Continue