PURPOSE: To accomplish an excavator solely dedicated for pit excavating, which particularly suits excavating forward a pit of large bore and large depth.

CONSTITUTION: Arrangement as per invention is composed of two boring units, which are equipped on an existing large-sized wheel jumbo etc., and a working platform 20 set at the cutting surface in pit excavation. On this working platform 20, two revolving arms 22, 22 are installed, which revolve around the platform 20 and whose stretching length horizontally from the platform 20 is made adjustable. One of the boring units 10 is installed at the foremost of one arm so that a drifter 11 can move downward vertically, while the other boring unit 0 is mounted at the foremost of the other arm so that another drifter 11 can move outward horizontally and downward vertically. The two drifters 11, 11 are connected with a hydraulic unit AU installed on the ground.