PURPOSE: To form a uniform hardened layer and prevent rising of the ground surface by pouring a solidifying material in the rotating territory of stirring wings at elevating the stirring wings.

CONSTITUTION: A fixed frame 8 is provided on the extreme end of the arm 7 of a crawler 6, and brought down into a weak ground 3 from the ground surface 12 up to a decided depth, while rotating stirring wings 2 around a horizontal shaft 1 provided on the yoked legs 8", 8" of the lower end. Then a solidifying material 4 is supplied from the lower end opening parts 10', 10' of a solidifying material supply pipe 10 provided on the upper parts of the yoked legs 8", 8", and it is poured in a rotating territory 5 and mixed by rotation of the stirring wings 2. Next the fixed frame 8 is elevated so as to perform the pouring and mixing up to the ground surface 12, and the ground in the rotating territory! 5 is hardened and improved by mixing of the material 4 therein. The solidifying material 4 poured in the territory 5 is uniformly mixed in the territory 5 by rotation around the horizontal shaft 1 of the stirring wings 2 and elevating sliding motion. Further mixture uneveness due to action of centrifugal force is made even by sliding motion in both longitudinal and lateral directions, and the solidifying material 4 receiving the centrifugal force is discharged outside the territory 5 so as to prevent rising of the ground surface 12.