PURPOSE: To treat a ground in short time and stably by setting a soil bearing capacity of a stabilization treatment ground by the amount of improving material, obtaining a necessary thickness and mixing and agitating, performing stabilizing treatment, and constructing a foundation on it.

CONSTITUTION: A groove 2 is dug in a ground 1, solidifying agent is sprayed on the bottom part 3 of the groove 2 and the ground is mixed and agitated, a foundation 5 as a stabilization treatment ground 4 is set on it, and soil is re-applied to the groove 2. In this case, soil quality and soil bearing capacity of the pre-improvement ground 1 are measured, and the width B of the stabilization treatment ground 4 is set with a foundation bottom part width (b) and a load on the foundation 5 taken into consideration. Also a mixing thickness H of soil quality improving material is obtained, using an equation, from a safety factor α, a foundation bottom part width (b), a treatment ground width B, a pre-treatment soil bearing capacity q0; a post-treatment soil-bearing capacity q1, and correlation function β between a mixing and agitating machine and a soil quality improvement material. Thus additional stabilization treatment is eliminated, stabilization treatment is performed in short time, and the construction of improved foundation can be performed efficiently and at low cost.