To make an excavating position accurate by performing intermediate milling to correct dislocation, and performing the final milling thereafter when a pilot hole is dislocated from a normal position when a shaft is excavated between an underpass and the ground surface.

A pilot hole 4 is bored by adding it while pushing out a drill rod 2 by an excavator arranged aboveground. When the pilot hole 4 is dislocated from a design position 5 when it reaches an underpass, a tip bit of the drill rod 2 is removed, and a milling bit 3b' having a radius not less than a dislocation quantity (d) is installed. Next, intermediate milling is performed while pulling up the rod 2, and a shaft 6' containing the design position 5 is excavated. A milling bit 3b" having a prescribed diameter is installed on the rod 2 set in a normal position (the design position) 5, and the final milling excavation is performed while pulling up the rod 2. Therefore, a shaft 6 according to the design position can be executed.