PURPOSE: To enhance the construction efficiency by a method in which a vertical pit is excavated by a small-width excavator, and while pulling the excavator along the pit wall by the wire of an excavator for vertical pit, a trench is excavated, vinyl sheets, reinforcing bars, and shape steels are set in the trench, and concrete is placed.

CONSTITUTION: A small-width excavator A consisting of a chain with claws is set on an excavator having a boom to be tilted 90° bilaterally, and a coupling plate 12 is set on the vertical intermediate of the excavator A and connected with a throttle wire 14 set on the excavator. A vertical pit is excavated by the excavator A and the excavator A is pulled laterally by the wire 14 to excavate a trench. In the trench, a waterproof vinyl sheet and reinforcing material such as reinforcing bars are set, and concrete is placed. An underground wall having excellent water-stopping effect can thus be constructed with a high efficiency.