To recycle a hardened material of soil as a soil material by rapidly hardening the soil such as rejected soil from a construction work site with a soil hardener within the range of pH without causing trouble in growth of plants.

This soil hardener comprises 50-70 wt.% of a blast furnace slag, 10-40 wt.% of magnesium oxide and/or a magnesium oxide-containing material expressed in terms of the magnesium oxide, 10-30 wt.% of a calcium compound and 5-50 wt.% of aluminum sulfate and/or polyaluminum chloride and/or an iron salt. The soil hardener is added and mixed with the soil to thereby rapidly harden the soil.