To considerably shortene a construction period by eliminating a need for compacting work by roller compaction, to considerably improve erosion-proof and earthquake-proof properties, and to reduce a burden on an environment by effectively utilizing construction by-products like soil produced in construction, construction sludge and concrete rubble.

There are used coarse-grained fractions 40 and soil mortar paste 50 which are prepared by mixing fine-grained fractions, water, and a hardening material to fill a gap formed by the coarse-grained fractions 40. The coarse-grained fractions 40 and the fine-grained fractions are collected from the construction by-products, and adjusted. For back-filling, (b) a tremie tube 20 is arranged in an excavated channel 10, (c) the coarse-grained fractions 40 are put into the excavated channel 10 using a putting means like a hopper 30, and (d, e) soil mortar paste 50 is placed while the tremie tube 20 is pulled out so that the gap formed by the coarse fractions 40 may be filled.