PURPOSE: To increase ground strength, by a method wherein water is added to fine iron powder contained in ground and high heat is generated and underground water is vaporized and the iron powder is turned into iron oxide due to high heat and soil is hardened by compressive force in case of expansion of the iron oxide.

CONSTITUTION: Vertical slots 2 are bored through ground 1, and when iron powder 3 which is previously mixed with water is contained in the vertical slots 2, then the iron powder 3 is oxidized and high heat is generated, and water around the vertical slots 2 is vaporized, and soil around the vertical slots 2 is heated. And compressive force is to be applied to soil around the slots, because piles 4 of ingot-like iron oxide of high strength and of diameter larger than that of the vertical slots 2 are formed as the iron powder 3 is turned into iron oxide 3' with cubical expansion.