To make it possible to conduct work for liquefaction countermeasure in a narrow place between an existing structure such as an existing detached house and a neighboring house silently and at a low price.

A foundation 12 immediately below a structure 10 is continuously surrounded by a wall body 14 and a lower end 14a of the wall body 14 is in a liquefaction layer 12b of the foundation 12. The wall body 14 is pressed downward in the foundation 12 around the structure 10 so that the foundation immediately below the structure 10 is continuously surrounded by the wall body 14. The lower end 14a of the wall body 14 stays inside the liquefaction layer 12b of the foundation 12. Preferably the lower end 14a of the wall body 14 is located at 1/3 to 2/3 of the depth of the liquefaction layer 12b of the foundation 12. The structure 10 is preferably an existing structure such as an existing detached house. The wall body 14 is preferably a lightweight steel sheet pile.