PURPOSE: To promote excavation efficiency by excavating underground hollows by an excavator in such a way that both ends of a tunnel are opened to a vertical shaft in a bedrock surrounding the vertical shaft connected to the ground surface.

CONSTITUTION: A vertical shaft 2 is board downward from the ground surface 7 until it reaches specific depth D. After that, an excavator is provided to a specific starting position P inside the vertical shaft 2, it starts in a direction perpendicular to the vertical shaft 2. Then, an excavating direction is gradually changed to form a generally annular tunnel 3 behind the excavator 9, and it is made to arrive at the vertical shaft 2. A plurality of tunnels 3 are formed in the shape of horizontal loops at specific depths, both ends 3a and opened to the vertical shaft 2, and underground hollows 6 having specific capacities in the tunnels 3 and vertical shaft 2 are made. According to the constitution, even if the pace of ground work is limited, the underground hollows can be formed.