PURPOSE: To prevent powder solidifying material from being stuck and solidified with the lapse of time to jet nozzle provided for the gas transporation and jetting of the powder solidifying material used for weak ground improvement so as to enable the jetting of the powder solidifying material as designed and to improve the durability of a device and a plant.

CONSTITUTION: A jet nozzle 4 is provided at the front end of the gas transporation 3 of powder solidifying material provided at a casing pipe 1, and a pure gas jet nozzle 14 is placed to face the jet nozzle 4 coaxially from the back part so as to assist the jetting of the powder solidifying material into soil by gas transportation from the back part by pure gas. The jet nozzle 4 is wiped in such a way as to prevent the adhesion and solidification of the jet nozzle 4, thereby impeding the diameter change of the jet nozzle 4 and preventing pressure loss. The gas transportation of the powder solidifying material such as cement supplied for soil improvement can be thereby performed as designed so as to maintain the reliability of construction, to shorten the construction period, to omit maintenance and to improve the durability of a device and a plant.