According to the NHK broadcaster, around 80 police officers together with the fire officials began working at the castle on Friday morning, focusing on the main hall that is believed to be the epicenter of the fire.
A major blaze broke out at the site, located in the city of Naha, shortly before 2:40 a.m. local time on Thursday (17:40 GMT on Wednesday). It has been put out only by 1:30 p.m.
The castle is part of a complex of monuments and buildings representing the history of the Kingdom of Ryukyu, which existed on the Ryukyu Islands between the 15th-19th century.
The popular tourist site was included in the UNESCO World Heritage list in 2000.
There have been no reported casualties from the blaze so far.