PROBLEM TO BE SOLVED: To provide a soft soil conditioner capable of providing necessary strength while keeping trafficability in a short time by mixing with soft soil, and capable of preventing effects by expansion after using, and to provide an improvement method of soft soil and an improvement method of soft ground by using the same.SOLUTION: There is provided a soft soil conditioner containing a first steelmaking slag subjected to an aging treatment and a second steelmaking slag not subjected to the aging treatment and having a smaller maximum particle diameter Dthan the maximum particle diameter of the first steelmaking slag D. There are also provided an improvement of soft soil and an improvement of soft ground by using the same.
