To effectively create a large-diameter ground improved body by rotating an injection rod for drilling, injecting pure water from an injection nozzle at a high pressure from the desired depth to bore a large-diameter hole, and injecting and filling a hardening agent at a high pressure at the time of a retreat.

An injection rod main body 11 is provided with a bit 18 at the tip section, a monitor section 19 nearby, first and second injection nozzles 12, 13, and a third injection nozzle 14 on the swivel side at a separate position for time differential boring. An injection rod 10 is rotated for boring to the upper end of the desired depth, then fresh waters 15, 16 are injected at a high pressure from the first and second nozzles 12, 13 while drilling, and fresh water or air is injected at a high pressure to the loosened ground from the third injection nozzle 14 to finish-excavate a large-diameter cavity section. A hardening agent is injected and filled at a high pressure from three injection nozzles 12-14 to harden the round while the injection rod main body 11 is rotatively lifted.