Rinnai Chasi, or Tsururaie, was a fort built by the Ainu people in Bihoro-cho, Hokkaido. “Rinnai” means “a place with piled earth” in the Ainu language. The fort was located on the hill, which commands a view of advance guard forts of Enkaru Chasi, Pira Chasi and Een Chasi. The place is said to have been the sanctuary where the guardian god of the village resided. As the Ainu people believed that if a person could not finish making something before the New Year’s Day, he will be possessed by the evil spirit named “Wen-kamuy,” they never failed to complete the task within a year. The fact that they constructed such a large-scaled structure within a year without using any machines tells us what a large power the Bihoro Ainu had grabbed at that time. The fort was a huge building made of cracked andesite stones popularly called “Ainu concrete” sealed with kneaded clay. It is unknown whether it was a watch fort or a defensive fort against enemies.