To improve soil, hardly changing the present situation of a floor slab, and besides, with reinforcing bars constructed in the soil.

A plurality of branch reinforcing bars 6 are radially arranged in the middle of a trunk reinforcing bar 2 in a cavity G from a through hole S1 cut in a floor slab S. A reinforcing member 1 in which these branch reinforcing bars are pivotably attached in an oscillating manner between the expanding direction and the upwardly overlapping direction is inserted with the branch reinforcing bars upwardly overlapping condition. The branch reinforcing bars passing through the through holes are opened in the expanding direction within the expanding direction in the cavity, and the grouting material is poured into the cavity from the through hole.