Abstract: A method of reusing a slurry in a ground improving method, wherein a hardening agent is injected into the ground, and wherein the muddy water (i.e., the slurry which is discharged) upon working is regenerated and again used as an injection material; namely, a hardening agent. By measuring the physical amount indicative of the permeability characteristic of the hardening agent contained in the slurry, an amount of hardening agent contained in the collected slurry (i.e., an amount of hardening agent to be collected) is calculated. Since the amount of collected hardening agent is calculated simultaneously with the collection of the slurry, the amount of hardening agent to be newly injected can be controlled in a real-time manner on the basis of the latest data (i.e., numerical value).