PURPOSE: To prevent the leak of cement mortar packed in an open space between the wall of a foundation pile and the outer wall of a runner plate by a method in which a hollow elastic material is set in the open space and expanded.

CONSTITUTION: A hollow elastic material 5 is set in an open space A between the wall B of a foundation pile and the outer wall c of a plate 1b and then expanded by the supply of compressed gas to close the open space A. Then, cement mortar CM is injected from a discharge pipe 3 provided to each plate 1b into the open space A, and after the cement mortar CM is hardened, compressed gas in the hollow elastic material 5 is released to the open air and a support pin 6 and a driving bar 7 are drawn out. Thus, without cement mortor CM being hardened, soil and sand layer D below the cement mortar CM can be excavated forwardly.