To solve the problem that when a water-soluble chelate chemical agent is used for treating a pollutant having a high heavy metal content, an amount of water is increased, handling of a treated matter is complicated, a soluble chelate flows out to an external part and heavy metal fixing ability is made insufficient.

Since a heavy metal treatment agent containing an iron (iii) dithiocarbamate complex of piperazine is hardly dissolved and has high reactivity with a heavy metal, a heavy metal pollutant having a high heavy metal content can be treated by mixing small amounts of a chemical agent and water and the heavy metal can be stably fixed for a long time.