A contaminated soil purification system for decomposing a contaminant in contaminated soil E. The contaminated soil purification system includes: injecting, into contaminated soil through a water injection well, an injection liquid containing a purification agent to decompose a contaminant or an activator to stimulate biological decomposition by the purification agent; a process of measuring a concentration of the purification agent or the activator in groundwater in an observation well or in a pumping well, the observation well or the pumping well being provided at a location spaced apart from the water injection well; and a process of, based on the concentration that has been measured, automatically controlling an addition amount of the purification agent or the activator to the injection liquid or automatically controlling a pumping amount of water from the pumping well.