PURPOSE: To reduce the quantity of excavated soil and sand carried to the outside of a pit by forming a double structure of an internal skin plate and an external skin plate in a cone-shaped section, a diameter of which is reduced in front, and forming a transfer path for excavated soil and sand between both plates.

CONSTITUTION: A shield jack 4 is elongated, each internal and external skin plate 10, 14 is pushed and propelled, and a part of excavated soil and sand are taken into a shielding machine by a screw conveyor 12. The greater part of soil and sand are guided by a cone-shaped section 11 at that time, and pushed into a transfer path P in the direction of the arrow B. The sectional area of an outlet section at the rear end of the path P is made smaller than the external skin plate 14 at that time, the density of soil and sand is increased during a time when soil and sand pass in the transfer path P and a section between an existing segment S and a bedrock G is filled with soil and sand. A high density layer D is formed.