To provide a chemical substance decomposition agent which effectively decomposes contaminating chemical substance, and to provide a method of cleaning a contaminated matter using the same.

The decomposition agent mainly consists of persulfates such as sodium persulfate, potassium persulfate, and silver ion is blended to this. Thus, the decomposition drug decomposes the chemical substance with a decomposition ratio much higher than a conventional chemical substance decomposition agent mainly consisting of persulfate and hydrogen peroxide. Moreover, when hydrogen peroxide is further blended, the decomposition drug rapidly improves its decomposition ratio of the chemical substance. Moreover, treatment of soil, groundwater, etc. using the chemical substance decomposition agent cleans the soil, groundwater, etc. very effectively.