PURPOSE: To selectively and exactly inject a grout into the ground by a method in which a pump is provided to each hose between grout sources facing a composite grout supplyer, and each pump is connected through a clutch to a drive system.

CONSTITUTION: In injecting a grout into soft ground 4, a controller 21 is operated according to a predetermined program, and by selectively operating the spool valves 20, 20' and 20" of the air cylinders 24, 24' and 24" of pumps 12, 12' and 12", the clutches 18, 18' and 18" are selectively turned in or out. For this reason, the pump selected by rotation of the motor 15 is operated, and a desired grout is supplied through a hose 11 from grout sources 9W11 to an injection tube and then injected into the ground. According to the program in order, the pumps are automatically selected and the ground is improved.