PURPOSE: To generate a requisite effect quickly by sinking a liquefaction generator into a soft soil to attain a hard layer while a pressure air and water are supplied to the liquefaction generator, and adding a material capable of hardening to the liquefied soft soil situated over the generator.

CONSTITUTION: A pipe network 14 is provided on a surface 24 of a soft soil point. Water and/or air from tanks 18 and 20 is supplied to the network 14 and allowed to flow out from a small hole in the network 14. Then the soft soil above and below the network 14 liquefies to cause the network 14 to sink. When the network 14 attains a rockbed 12, the avobe-situated soil further liquefies with continued supply of water and air. Stone splinters 25 are cast into the liquefied soft soil, and cement from a tank 22 is supplied into the soft soil from the network 14.