A subsurface soil purification method including: warming an activator liquid, for stimulating decomposer microorganisms that decompose a contaminant in subsurface soil, to a higher temperature than a groundwater temperature, and feeding the activator liquid into the subsurface soil by injecting the activator liquid into an in-ground injection well; warming an activator liquid, for stimulating decomposer microorganisms that decompose a contaminant in subsurface soil, the decomposer microorganisms being infused in the activator liquid, to a higher temperature than a groundwater temperature, and feeding the activator liquid into the subsurface soil by injecting the activator liquid into an in-ground injection well, or warming a purification liquid for decomposing a contaminant in subsurface soil, to a higher temperature than a groundwater temperature, and feeding the purification liquid into the subsurface soil by injecting the purification liquid into an in-ground injection well. The subsurface soil purification method also includes forcing air into the injection well, and feeding the air into the subsurface soil from a position in the injection well that is lower than a position in the injection well for feed-in of the activator liquid or the purification liquid.