impellers when said agitator impeller reaches the surface of said bottom supporting foundation 1 said motor 21 begins to rotate in the reverse direction and said agitator impellers are elevated one after another conversely when said agitator impeller reaches a certainly predetermined height it is again lowered said weak soil layer is thus continuously treated and stabilized up to a certain height from said bottom supporting foundation 1 and over a certain width and length by repeating the above operations together with moving said operation boat since the lower ends of base plates 19 of each agitating unit which is herein elevated and lowered are not at a constant level due to differences of the supporting force of said bottom supporting foundation 1 it is preferred that the upper limits of movement of said base plates 19 are made constantly fixed because said upper ends determine the upper surface of the ground to be treated and hardened the upper and lower limits of movements of said base plates 19 are controlled by changing the direction of revolution of said motor 21 in accordance with the value measured by said ground pressure detecting device and with torque change in the other means there is a further simple means to control the upper limits of movements of said base plate 19 i e limit switches mounted at a fixed position on frames 18a and 18b along which said base plate 19 is elevated and lowered thereby the upper limits of said base plate movement are effectively controlled when said base plate comes into contact with said limit switches when said base plate is elevated the upper face of.