Shuri Castle, a 14th century World Heritage site in Okinawa, has been ravaged by the blaze, which was still burning early this morning.
A popular tourist attraction, the wooden structure was constructed as the royal castle of the Ryukyu Kingdom.
After World War II, the castle served as a campus for Okinawa's largest public university until the mid 1970s.
(Image: EPA)It was designated a World Heritage site in 2000.
In addition to the main hall, the north and south halls have been destroyed, NHK said.