This is the only unjo-ya (trading post office) in Hokkaido. It was established as a base for the Wajin to manage Ezo and as a facility for trade with and care for the Ainu. It is a nationally designated cultural property.

During the Edo period, Unjoya buildings were built in various locations throughout Ezo as business bases for merchants who were contracted to trade with the Ainu people by the Matsumae Domain. The former Shimo Yoichi Unjoya is the only remaining Unjoya that was built in 1853 by Takeya Hayashi Chozaemon, a contractor for the Yoichi location, and its long, gabled, stone-set roof, lattice windows, and paper shoji doors are in the same style of design as fishermen's houses in the Hokuriku region.

The interior space is spacious, with thick pillars and beams visible between the wooden boards, and many of the original materials are still in use. The building has a width of 40 meters, a depth of 16 meters, a floor area of 534 square meters, and 213 cubic meters of lumber. In the rooms on both sides of the corridor, dolls of the manager, Matsumae samurai, and other characters sent by the location contractor are on display.