PURPOSE: To prevent the adhesion of gelled substances to an injection pipe by a method in which an expandible bag is attached to the nozzle port of the inner tube of a multiple injection tube and the nozzle port of the outer tube is opened or closed by expanding or shrinking the bag by the supply or discharge of a liquid.

CONSTITUTION: When boring water is sent from the flow path 2a of an outer tube 2, the water flows out by pushing down a ball valve 10 through the lower nozzle port 9, and thereby the ground is excavated. Then, when a water glass solution or a mixture of water glass and a reactant is sent from the flow path 2a and a aqueous reactant solution is sent concurrently from the flow path 1a of the inner tube 1, an expandible bag 3 is expanded by the pressure of the fluid to close the flow path 2a. Further, the aqueous reactnat solution expands the sleeve 7 of rubber from the nozzle port 6 of the inner tube, spouts to the flow path 2a, and is mixed with the water glass solution or the mixture of water glass and reactant. The mixture is jetted from the upper nozzle port 8 of the outer tube 2 to the outside of the outer tube 2 and injected into the ground.