To solve a problem wherein high voltage flows to timbering to cause danger if electric discharge is carried out with the base end of a discharge electrode in contact with timbering.

An electric discharge hole 4 extending in the proceeding direction of excavation is formed in a tunnel excavation object part, and the discharge electrode is provided in the electric discharge hole to generate shock wave by electric discharge at an electric discharge part of the discharge electrode and to excavate a tunnel by breaking the tunnel excavation object part with the shock wave. In this case, the timbering 93 for supporting the natural ground is provided inside the peripheral edge of the hole finished with excavation, and the electric discharge hole is formed in a peripheral edge part 96 of a working face 94 located on the excavation proceeding direction side from the timbering. A bendable discharge electrode 1 is used as the discharge electrode installed in the electric discharge hole to carry out electric discharge, and after the discharge electrode is bent and separated from the timbering, electric discharge by the discharge electrode is carried out within the electric discharge hole to break the tunnel peripheral edge part.