It is the Kure Navy base Commander-in-Chief official residence. It is the so-called "Admiral's house." This house does not appear in the anime and game, however the Admiral lives there.
This official residence was built in 1905. The front is Western-style, and the back side is Japanese style. Former Commanders-in-Chief lived there too. The roof is scale-shaped tiles. Japanese leather paper covers the wall of the Oval Office. It is a very valuable building.
The admission is 250 yen.