PURPOSE: To eliminate loss of soil discharge quantity due to reverse rotation of a screw in the case of carrying out foundation improvement by way of injecting a solidifying agent and to certainly carry out soil discharge in required quantity.

CONSTITUTION: Holding blades 4 to hold drilled soil taken in on screws 3 at the time of reverse rotation are axially supported free to rotate around a vertical shaft or fixed between the screws 3 at least of one any pitch of the screws 3 projectively provided on the outer periphery of a hollow pipe 2. The holding blades 4 avoid falling of the drilled soil from on the screws 3 and draw up the drilled soil on the ground as the hollow pipe 2 is drawn out at the time of reverse rotation of the hollow pipe 2.