PURPOSE: To eliminate the dispersion of improved strength in the diametrical and depthwise directions of soft ground by attaching plural diffusive blades to diffuse an improving agent to the diametrical direction to the periphery of a valve body attached to the lower end of a screw feeder.

CONSTITUTION: Plural diffusive blades 10 to diffuse an improving agent, e.g., quick lime, etc., to the diametrical direction are attached to the periphery of a valve body 9 attached to the lower end of a screw feeder 8 whose spiral pitch is increased toward the discharging direction. A hollow drive pipe 6 and a stirring shaft 7 are each connected to separate drivers 3 and 4 whose rotation number difference can be optionally regulated. Since the pitch of the screw feeder 8 is increased toward the discharging direction, better discharge of soil can be attained and soil improver of uniform diameter can thus be constructed by the expanding blades 10.