This entire building is constructed of thick slabs of granite, including both external and internal walls, the floor and the ceiling. This solid structure is the sole such example of a stone gunpowder storehouse existing today. Having learned a lesson from the large explosion of the wooden gunpowder storehouse in 1660, the Tokugawa government remodeled it into the present stone building. A historical document which recorded the date of the construction was found in recent years, and it confirmed that this warehouse was restored in 1685.