Thick smoke could be seen billowing out of the Shuri castle in Naha, Okinawa prefecture, where a massive blaze engulfed the 14th-century structure at about on Wednesday according to firefighters, who struggled to contain the flames, reported Efe news.
The flames were put out about nine hours after it broke out.
About 30 firetrucks arrived at the site, where the fire had spread through about 4,200 square meters of the compound, according to Japan's national broadcaster NHK.
There were no reports of injuries, but 30 people were temporarily evacuated. The cause of the fire, which allegedly started near the pavilion, is being investigated.
One of the firemen who took part in extinguishing the flames had to be admitted to a nearby hospital due to dehydration.
Since Sunday, a festival, that recreated ritual scenes from the era when it was built, was being held at the castle, and staff were immersed in the preparations on Wednesday night.
The castle was built in the late 14th century and is an important tourist destination in the region, having been declared a national treasure in 1925.
It was destroyed in 1945 during World War II's Battle of Okinawa, but its pavilion was rebuilt in 1992 along with the rest of the compound, which was reopened as a national park.
UNESCO declared the Shuri castle a world heritage site in 2000 along with the nine places, ruins, and monuments previously belonging to the Ryukyu Kingdom, which became part of Japan's Okinawa islands in the 19th century.
( With inputs from IANS )