A fire early on Sunday gutted a historic wooden building which was the former residence of Shigeru Yoshida, who served as Japanese premier in the 1940s and 50s after World War II, an official said.
Yoshida is a grandfather of incumbent Prime Minister Taro Aso, who said he was saddened by the blaze.
"It was a memorable place for me," he said in a statement about the house in the coastal resort town of Oisomachi, southwest of Tokyo, where he reportedly spent his early childhood.
The wooden building, made of cypress, was built in 1884 by Yoshida's father-in-law and was used for a Japan-US summit meeting in 1979 when then US president Jimmy Carter visited.
"The fire completely gutted the main building as well as many trees in the garden and scorched annexes, too," a local official said.
"We still don't know what caused the fire," said the official. "We cannot rule out the possibility of arson."