PURPOSE: To obtain a high-strength and dense solidified layer of the ground by a method in which the first solidifying liquid having a comparatively longer hardening time is injected from the lower end of the main tube and the second solidifying liquid having a comparatively shorter hardening time is injected from the upper part rather than the lower end of the main tube.

CONSTITUTION: The first jet nozzle 18 is provided at the lower end of the main tube 11, and the second jet nozzle 9 is provided at an upper position a given interval from the jet nozzle 18. In the main tube 11, the first pathway 21 through which the first solidifying liquid A1 having a comparatively longer hardening time is passed and the second pathway 22 through which the second solidifying liquid A2 having a comparatively shorter hardening time is passed are provided. The liquid A1 is injected from the lower end of the main tube 11, and the liquid A2 injected from the place above the lower end of the main tube 11. The liquid A2 so injected hardens in a short time to form an auxiliary layer 36. The liquid A1 is ready to diffuse to the surrounding area until hardened because it takes a longer time to solidify. Since the diffusion of the liquid A1 is suppressed by the auxiliary layer 36, the dilution of the liquid A1 is prevented and thereby a solidified layer 37 having a high strength can be obtained.