A fire breaks out at the UNESCO's World Heritage site, Shuri Castle, in the southern prefecture of Okinawa on Thursday morning.
No casualty has been reported so far. An operation to douse the fire is underway, state broadcaster NHK reported. Five firefighter trucks are at the spot.
Police said that the main hall and a nearby building have burned to the ground.
The castle, built about 500 years ago by the Ryukyu Dynasty, was designated Japan's national treasure in 1933.