To reduce a constructing quantity of earth-retaining by constructing a shaft between a pair of earth-retaining walls constructed in the ground and hauling a shield excavator therein to excavate a hole for upper and lower floors and excavating the soil between the upper and lower floor slabs after these floor slabs have been formed.

A pair of right and left earth-retaining walls 10 are constructed in the ground from a road face. A starting shaft 15 and an arrival shaft are formed at the ends of the earth-retaining walls 10. Several small shield excavators are parallelly arranged in the starting shaft 15 to horizontally start in the lower floor part 19 to excavate the ground. In the same way, the upper floor part 24 is also excavated and segments with supports 21 are installed to support the natural ground 22. Reinforcements are arranged in the tunnel after the primary lining to connect them to the earth-retaining walls 10 and then, concrete is placed to construct the lower floor slab 27. After the upper floor slab 28 has been constructed likewise, the soil within the ground 29 is excavated and decorative walls 31 or the like of the side walls are finished and then the shaft 15 is filled back. The upper layer is not opened and hence, earth-retaining walls are not required and also a deep embedment is not required for the earth-retaining wall of the lower layer part and hence, a constructing quantity of the earth-retaining wall can be reduced.