To efficiently construct an anchor for disaster prevention (fall prevention) even in a stratum of loose density where its surface layer easily falls.

A downhole type excavating device comprises a moving mechanism (a frame, a guide rail and a moving body), an excavating motor provided at the moving mechanism, and a predetermined air hammer drill 6 connected to the excavating motor. In excavating the surface layer b using the downhole type excavating device, a predetermined casing 91 and a screw auger 92 disposed in the casing are connected to the excavating motor of the device to excavate, and when excavating down to a predetermined rock bed stratum, the screw auger 92 is removed while leaving the casing 91. The air hammer drill 6 is mounted to the excavating motor, and further a predetermined downhole means is used to continuously form an excavated hole B. A predetermined tendon C is inserted in the excavated hole B with the surface layer side protected by the casing, and cement milk D is injected to construct the anchor.

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