PURPOSE: To raise the economy of construction work by a method in which excavation is made to a given depth and then expanded excavation is made by an excavating rod, a solidifying fluid is injected, the excavating rod is removed, and a pile tipped with bag is inserted into the pit and the fluid is injected.

CONSTITUTION: While turning an excavating rod 1 with an excavating bit 5, a pit is excavated to a given depth by jetting an excavating liquid and expanded excavation is then made by high-pressure jet. A solidifying fluid C is injected, the rod 1 is removed, and a precast pile 10 tipped with a cylindrical bag 11 is inserted into the muddy pit K. Normal precast pile 12 is then connected to the pile 10 with the bag 11 and settled to a given position. The fluid C is then injected through an injection port formed in the pile tip into the bag 11 to expand the bag 11.