PURPOSE: To prepare title conditioner which is excellent in the preparation eficiency and the uniformity of conditioning degree, by providing a water-absorptive degratable cylindrical bag packed with sand and quick lime inside a water-resistant and -permeable and expandible cylindrical bag.

CONSTITUTION: The upper cover 8 is placed on the casing 6 with the bottom cover 7 and then the casing 6 is struck down to a desired depth into soft ground while supplying compressed air into the casing 6 through the compressed air supply tube 9. Then, the upper cover 8 is removed and then the soft ground conditioner A', consisting of the water-resistant and -permeable expandible cylindrical bag 1 in which the water absorptive degradable unit cylinder 4 filled with the quick lime 3 is arranged in the central axle direction to form the central part B, is inserted into the casing. Then, the sand hopper 10 in connected to the upper end of the bag 1 and then sand is filled in between the inner wall of the bag 1 and the central part B to form the conditioner A consisting of quick lime and sand. Then, the upper cover 8 is place on the casing 6 and then the casing 6 is drawn out while supplying compressed air to leave the conditioner A behind.