This is the only existing Unjōya within Hokkaido. It is a building that was established as a facility to conduct trade and fostering with the Ainu, as a base for the management of Ezochi by Wajin people. Nationally designated cultural property.

The Unjōya was a building that merchants who undertook trade with the Ainu people, which the Matsumae Domain was carrying out during the Edo period, made their management base, and they were built in various places in Ezochi. The Former Shimo-Yoichi Unjōya is the only existing Unjōya, built in Kaei 6 (1853) by Takeya Rin Chozaemon, who was the contractor merchant for the Yoichi trading post, and its gable roof with entrance on the gable side, long stone-weighted roof, lattice windows, and paper shoji doors are of the same system of design as fishing house architecture in Hokuriku.

The interior space is wide, and the pillars and beams looked up at from the wooden floor room are thick, and many of those structural members are using the original ones as they were at that time. The frontage reaches 40 meters, the depth 16 meters, the building area 534 square meters, and the amount of timber used reaches 213 cubic meters. In the tatami rooms arranged on both sides of the corridor, customs dolls of managers dispatched by the trading post contractors and Matsumae domain samurai, etc., are displayed.