To execute a ground improvement work at a great depth at low cost with high efficiency based on a dry construction method.

A tube-like casing 9 is pressed into the underground area where a powder-made ground improvement material 11 is charged thereinto. After the powder is charged, the casing 9 is pulled out and the ground improvement material in the casing 9 is left over underground in the form of a post. Then, an agitation rod with an agitation blade whose agitation blade side D is larger than the post size (d) is advanced into the post-like ground improvement material 11 from above while the agitation blade is being turned, thereby mixing and agitating the improvement material 11 together with the earth and sand in the surrounding area. Then, the agitation rod 10, while it is being turned inversely, is pulled out, thereby forming a post of a pile 12 mixed with the improvement material 11 and the earth and sand in the surrounding peripheral area.