To provide a foundation for a post minimizing the generated quantity of surplus soil, easy to construct and reusable separated from a fence body (the post).

The foundation 2 for the post has a body divided piece 40a provided with a bottom plate and side plates forming three faces and provided with an upper strip member 50a and a lower strip member 60a fixed straddling the facing side plates, and a body divided piece 40a provided with an upper strip member 50b and a lower strip member 60b. The upper strip member 50a and the upper strip member 50b are connected by a bolt 8b, and an intermediate part of the post 9 is gripped between both members. The lower strip member 60a and the lower strip member 60b are connected by a bolt 8b, and a lower end of the post 9 is gripped between both members. Excavated soil is backfilled in a hollow body 400 of square pole shape formed by the body divided piece 40a and the body divided piece 40b by such a connection.