To improve workability by installing an excavating edge into a cylindrical casing through a support member rotated together with the casing and excavating the while surface in the casing.

An internal-surface excavator 13 is mounted detachably to the lower section of a cylindrical casing 11, to which a plurality of cutting edges 12 are set up along the peripheral direction of a lower end section. The rod-shaped excavator 13 is fitted into the lower section of the casing 11 while being passed through the center of the casing 11, and excavating edges 14 are projected to one side in the peripheral direction of the lower section. When the casing 11 is rotated, the excavating edges 12 excavate a ground in a circular groove along the casing 11 while the excavating edges 14 excavate the ground on the whole surface on the inside of the casing 11. Accordingly, the efficiency of excavation can be improved because the ground in the cylindrical casing is excavated by the internal-surface excavator, and cost can be reduced because various kinds of hammer crabs need not be prepared in conformity with a shaft diameter, and vibrations and noises can be lowered because the hammer crabs need not be dropped freely, and general-purpose properties can be enhanced by using a clamshell.