Disclosed is a soil improvement device which has a nozzle provided with a blow hole of compressed air on a pipe tip part connected to a compressed air supply device and ejects the compressed air through the blow hole to excavate soil in the form of a hole, and is characterized by employing the nozzle so configured that the maximum outer diameter of a nozzle outer peripheral part bulges out compared to the pipe outer diameter such that (the area of a circle calculated by employing the nozzle maximum outer diameter as a diameter - the area calculated by employing the pipe outer diameter)/ the area of the nozzle blow hole is at least 5, and positioned on substantially the same line as a substantially linear pipe directed to the excavation direction.
