The Saiguden (祭具殿, Ritual Equipment Temple) is the ritual tool shed behind the Furude Shrine. The shed is always locked, and very few people have access to the key. This is because it is filled with a variety of torture implements that had been used over the course of the years in the name of Oyashiro-sama.
Some stories hold that these tools were used to punish people who broke the rules or caused trouble for the Three Great Houses of Hinamizawa. Others imply that they were used as tools for cannibalism, as the people from the village are believed to have partially descended from man-eating demons. There is even a theory that it may hold an alien being. Whatever its inital purpose was, it is strictly off limits and entering it can bring down the curse of Oyashiro-sama on whoever dares. In later story arcs it is revealed that Tomitake Jirō, Takano Miyo, and in some arcs Sonozaki Shion and Maebara Keiichi enter the shed on the night of the Watanagashi Festival.
According to TIPS, Furude Rika, the head of one of the Great Houses, once asked to have the lock changed to a simple lock because the original was difficult to open. Kimiyoshi Kiichirō, another head, complied and had the lock changed to a more simple type. It is speculated that because of this change, Tomitake was able to pick and open the lock, breaking into the shrine. It is also the source of the villages belief that they were the cause of the illegal entering and the reason for their own disappearance, as it would't have been broken into if the lock was the original heavy version, and thus Oyashiro-sama took them as punishment.