To positively improve floatation resistance of an underground structure even in a shallow ditch with a thin soil covered layer regardless of the structure of an excavated ditch.

A ditch 3 excavated without timbering is formed in a first process. In a second process, a pipeline 2 is disposed on a bottom face 5 of the ditch 3 excavated without timbering, and sandbags 6 are arranged between the pipeline 2 and the wall surfaces 3A, 3B of the ditch excavated without timbering. In a third process, both ends in the ditch width direction of a geotextile 10 are left over to put the geotextile 10 along the pipeline 2, the support ground C and the inner side faces of the sandbags 6. In a fourth process, a backfilling material 7 is supplied, and loading of the sandbags 6 and the supply of the backfilling material 7 are repeated to supply the backfilling material 7 up to a predetermined height L1. In a fifth process, both end leftover parts 10B, 10C of the geotextile 10 are opened and connected to wrap in the backfilling material 7 surrounded by the geotextile 10. In a sixth process, the backfilling material 7 is backfilled onto the geotextile 10 to backfill the ditch 3 excavated without timbering.

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