To provide a soil conditioner which can give treated soil of good long-term strength and to provide a soil conditioning method.

The soil conditioner contains gypsum hemihydrate and blast furnace slag and is characterized in that the mixing ratio of the gypsum hemihydrate to the blast furnace slag is 50:50 to 70:30 by weight and that 0.1 to 2.0 wt.% slaked lime is added to the blast slag. In the soil conditioner, the silicates of Al and Ca in the blast furnace slag harden over a long term by a pozzolanic reaction. It is considered that long term strength is improved because the slaked lime together with the sulfo groups of the gypsum catalyzes the reaction.