To infer the strength of improved ground which is an object improved by chemical injection using an active-silica-based chemical, from its early age after the improvement, with accuracy.

A vibrator sensor capable of generating and receiving compressional waves and shear waves is installed, in ground to be improved by injecting the active-silica-based chemical into the ground. By using a compressional-wave velocity obtained by this vibrator sensor, and applying it to a regression curve representing relation between compressional wave velocity and uniaxial compressive strength obtained by an indoor test previously, the uniaxial compressive strength of the improved ground is inferred.