PURPOSE: To continuously form sand piles by a method in which plural sand storage hoppers are provided, and while sand is charged from one sand storage hopper to a sand pile-forming hollow cylinder, sand is supplied supplementally to the other sand storage hopper.

CONSTITUTION: A sand pile-forming hollow cylinder 11 is penetrated into soil layer to be improved, sand is stored in the hollow cylinder 11, and while discharging the sand and applying a vibration to the cylinder 11, the cylinder 11 is drawn out to form a sand pile P. Plural sand storage hoppers 21 and 21' are provided, and while sand is charged from one hopper 21 into the cylinder 11, sand is supplementally supplied to the other hopper 21'. Then, while sand is charged from the hopper 21' into the cylinder 11, sand is supplementally supplied to the hopper 21. By repeating such operations, sand pile P is continuously formed.