To purify contaminants in the ground by diffusing iron powder together with water or muddy water into the contaminants in the ground.

A prescribed quantity of iron powder is mixed with water or muddy water to adjust drilling water. Next, from a nozzle 1 opened at the head of a bit 12, the drilling water is jetted to perform boring in the ground. On boring, iron powder contained in the drilling water is injected together with water or muddy water. Or, on boring, a rotary tool (reamer) is driven to agitate the water or muddy water and iron powder and to incorporate the iron powder into the stratum. In this way, contaminants in underground water or soil can be purified.