To provide an in situ purification method of a polluted ground or polluted underground water utilizing the Fenton reaction, which enables the purification of the polluted soil or the polluted underground water in situ over an extensive range.

This in situ purification method of the polluted ground or the polluted underground water utilizes the Fenton reaction, and supplies a solution containing divalent ferrous ions and a hydrogen peroxide solution together with a water-soluble alcohol to the polluted ground or the polluted underground water fouled by an organic pollutant. Thus the purification of the polluted ground or the polluted underground water is performed at a source site.