PURPOSE: To improve the workability by protecting an underground buried object by way of suspensing it at the time of drilling a ground, filling a fluid backfill between the underground buried object and an underground structure after constructing the underground structure and backfilling the whole by way of removing a suspension protector after the fluid back-fill is hardened.

CONSTITUTION: At the time of drilling a ground in the circumference of an underground buried object X, the buried object X is held by a receiver girder 3 and the underground structure K is constructed. Thereafter, a back-filling material MC of drilled soil mixed with cement and water is filled on the structure K, a first back-filled ground 6 is constructed, and a primary backfilled layer 6b is constrcuted with the drilled soil on the left and right of the first back-filled ground 6. Subsequently, the buried object X is suspended by a sling, and after the girder 3 is removed, a secondary back-filled ground 7 is constructed with the drilled soil on the left and right of the ground 6. Thereafter, a second backfilled ground 7 is constructed by using the back-fill MC on the ground 6. Additionally, a receiver proterctor 8 is construted between the buried object X and the ground 7, and after the sling, etc., are removed, a pit sand back-filled layer 8c and a third back-filled layer 9 of the drilled soil are constructed. Consequently, it is possible to easily back-fill by using the drilled soil.