The Sonozaki Main House (園崎本家 sonozaki honke) is a location in Hinamizawa. It was first introduced in Watanagashi-hen.

Mion lives here with her grandmother to train to become the next successor to the Sonozaki Family.

The main house contains the Underground Saiguden or Underground Ritual Storehouse, sometimes referred to simply as "The Basement," it is where a large amount of torture devices are stored. Supposedly, according to Takano's research, it is here that the Watanagashi ceremony used to be performed in the days of Onigafuchi.

Keiichi remarks that the tools stored here are significantly more well maintained than their counterparts stored in the Saiguden in the Furude Shrine.

Just as Takano suspects, the Underground Saiguden has a place where ceremonies are performed by the main family, with places to sit for the branch families to watch. This place is shown used once in the games, when Shion is forced by Mion to rip off three of her own nails as punishment for trying to defend a murderer. It can be assumed that Mion, Akane and Oryō all performed their respective punishments here.

The Underground Saiguden also has a number of holding cells, possibly used in the past to contain sacrifices before ceremonies, these cells were used by Shion in Meakashi-hen. One such cell has a hidden ladder, that leads to a well where bodies used to be disposed of.

This place also has numerous tunnels and secret paths that lead to the outside world, and can be used by the Sonozaki Family to escape in case they ever get raided by the authorities. Only the Head of the Family knows how to navigate the Underground.