To enhance economical efficiency by constructing a land slide protection wall by contacting a pile and a pile with other in a water stop unnecessary depth directional section, and constructing the land slide protection wall by partially superposing cross sections of the pile and the pile on each other in a water stop necessary section.

In an earth's surface side clay layer composed of the water impermeable ground, excavation is performed still in a diameter of a screw head arranged on the tip of a rotary rod. Cement milk or mortar is delivered from the tip of a pipe arranged inside of the rotary rod, and is mixed with surrounding earth and sand, and a column-rowed land slide protection wall of soil cement or mortar is constructed in a condition where a pile and a pile are contacted with each other. When reaching a water permeable sand layer by penetrating through a clay layer, excavation is performed by expanding an expansible blade having a diameter larger than the screw head. The cement milk or the like is delivered from the tip of a pipe arranged inside the rotary rod, and is mixed with surrounding earth and sand, and the column-rowed land slide protection wall of soil cement or the like is constructed in a condition where cross sections of a pile and a pile are partially superposed on each other.