PURPOSE: To permit a stright pit to be excavated by a method in which an auter casing is turned in the first direction, and an inner rod in the casing is turned in the second direction as the opposite direction to the first direction to excavate surface soil.

CONSTITUTION: An oil-pressure motor 4 is actuated to rotationally operate an auter casing 1 in the direction of (a). Also, at the same time as an inner rod 10 is turned in the opposite direction (b) of the direction(a) with almost the same turning number as that of the casing 1, compressed air and water are supplied to the center hole of the rod 10. An air hammer 8 is driven by the compressed air to an excavating direction and while vibrating the metal bit 2 of the casing 1 and the hammer bit 9 of the rod 10 by application of impact in the direction C, surface soil is excavated. The slime is further crushed and discharged with air and water from an outlet 6.