PROBLEM TO BE SOLVED: To provide a recycling processing method that insolubilizes and stabilizes fluorine in the carefully selected soil extracted from sediment-based mixed waste, and also insolubilizes hexavalent chromium, and thereby the elution of the fluorine and hexavalent chromium can be prevented over a long period of time even under the mild acidic condition such as an acid rain.SOLUTION: A recycling processing method of the carefully selected soil extracted from sediment-based mixed waste is characterized as follows: a phosphorus-containing compound is applied to the carefully selected soil extracted from the sediment-based mixed waste further, a first mixture obtained by applying water and mixing it is cured insolubilization processing of fluorine in the carefully selected soil is carried out and the carefully selected soil subjected to the fluorine insolubilization processing is obtained a humic substance and a ferrous compound are applied to the carefully selected soil subjected to the fluorine insolubilization processing the phosphorous component reacted with the fluorine is made to adsorb in the humic substance by stirring processing and the hexavalent chromium is insolubilized by the ferrous compound to make the recycling processing carefully selected soil. Also, an insolubilization agent for recycling processing of the carefully selected soil extracted from the sediment-based mixed waste is formed by containing a ferrous humus composite being the composite of the humic substance with the ferrous compound.
