PURPOSE: To inhibit the deformation and stress of a landslide protection wall at small values, and to decrease the number of the stages of all timberings at the time of large depth by lowering two stages of the upper and lower timberings while the landslide protection wall is suppressed alternately in a measuring worm manner and advancing excavation.

CONSTITUTION: Primary excavation is executed in an extent that the deformation of a landslide protection wall 1 is not increased, and two stages of upper and lower timberings A, A' are installed into an excavation section. Secondary excavation is conducted, and the lower-stage timbering A is brought down near an excavation surface B by a jack and fixed. A normal timbering C1 at a first stage is mounted to the upper section of the upper-stage timbering A', and the upper-stage timbering A' is lowered up to the upper section of the lower-stage timbering A by the jack. Excavation, the descent of the lower- stage timbering A, falling near the lower-stage timbering A of the upper-stage timbering A', and the installation of the normal timbering to the upper section of the upper-stage timbering A' are repeated, and excavation is advanced.