Abstract: The invention provides a jet-mixing method for casting a pile of predetermined diameter by thrusting or feeding into the ground a rod provided at its lower end with a first jet nozzle section for jetting ground improvement medium and a second jet nozzle section for jetting reactant in substantially the same direction as the jet direction of the first jet nozzle section, and provided above the first and second jet nozzle sections with a third jet nozzle section for mixing and jetting compressed air and water in a direction opposite the jet direction of the first and second jet nozzle sections, jetting compressed air and water within a predetermined range while simultaneously rotating and upwardly extracting the rod from the ground, thereby jet-cutting a ground improvement region into a cylindrical shape, discharging mud generated by the cutting operation to the ground surface through a bored hole, and jetting the reactant from the second jet nozzle section and jetting the ground improvement medium from the fir