PURPOSE: To solidify upper and lower ground layers with uniform strength by improving these grounds three-dimensionally by a method in which plural kinds of improving agents are used as they are or in blended manner for injecting and stirring in the soil of the ground.

CONSTITUTION: Plural kinds of improving agents, corresponding to the properties of soft ground, are stored in plural tanks or silos 11 and 11'. The improving agents are taken out of the silos 11 and 11' through conveyers 10 and 10', mixed together in a mixer 13, and sent under pressures to an injecting and stirring device 4 through a pneumatic supplyer 9 and a switch valve 6. The improving agents corresponding to the properties of the ground can thus be integrally mixed with the natural ground for improvement.