To obtain a ground improving agent solidifying ground rigidly and economically preventing elution of hexavalent chromium from the solidified ground by a simple method.

This ground-improving agent is characterized by comprising a mixture of a cement-based solidifying agent and powder of coal or lignite. The coal is brown coal or anthracite. Blast-furnace slag, ferrous sulfate, quick lime or slaked lime can be included in the ground-improving agent. The method for its production comprises applying the cement-based solidifying agent including coal or lignite powder to soil or comprises adding the powder of coal or lignite to the cement-solidifying agent and mixing the resultant mixture with the soil.