To detect the soft and hard places of the ground simply, quickly and accurately when a method of ground stabilizing construction, etc., are executed by detecting the fluctuation of the resolving torque of a nozzle bit or a rod during the injection operation of a fluid.

A nozzle bit 11 is installed to a rod for a device used when a method of ground stabilizing construction is executed while a strain gage is mounted at the proper place of the rod. The rod is inserted into the ground, and a fluid is injected from a nozzle 12 while the rod is rotated at high speed. Since the nozzle bit 11 receives rotational reaction at all times during the injection operation of the fluid, revolving torque applied to the nozzle bit 11 differs in a hard stratum section and a soft stratum section in the ground. Accordingly, the fluctuation of revolving torque applied to the nozzle bit 11 or the rod is detected by the strain gage, and the variation is displayed on a graph and the soft and hard states of the ground at a place where the nozzle bit is inserted, are detected.