To construct a column having large strength, by constructing a small diameter improved earth pile in the first place, and performing operation for increasing a diameter later without discharging excessive earth, when constructing a columnar ground improving layer of a cast-in-place pile shape.

An excavation head 10 is supported by a work device, and excavates and agitates earth by being inserted into the ground. A device for supplying a solidifying material is connected to a driving mechanism. An excavation blade part 20 is fixed to and held by a lower end part of a rod 11, and is connected to its upper part via a joint 12 capable of engaging and disengaging an upper flight member 14. After excavating a small diameter vertical hole by rotating a rod member in one direction, the solidifying material is filled in a lower part while pulling up the excavation blade part. The upper flight member 14 is blocked up by leaving in the upper part, and an improved earth column having large strength is constructed by mixing earth in a hole and the solidifying material.