PURPOSE: To excavate an underground section horizontally and continuously in a short term of work.

CONSTITUTION: The ground is excavated in the horizontal direction by using an earth auger 10, in which a plurality of pawls 16 for excavation are mounted on the outer circumference of a screw 14 spirally wound around a shaft 12, and excavated soil and sand are discharged onto the ground. Water 36 and cement 38 are mixed with excavated soil and sand 34 discharged onto the ground, thus manufacturing a slurry-like self-hardening mixture 42. The mixture 42 is poured into the side of a seal member 30. Accordingly, the pressure of the slurry-like mixture 42 works to the seal member 30 in the direction shown in by arrow 44, thus propelling the earth auger 10 to the side, then advancing excavation in the horizontal direction.