A cleansing processing agent that is able to remove harmful substances contained in a variety of waste materials and a cleansing method are disclosed. Specifically, there is disclosed a cleansing processing agent containing a polymer having at least one of styrene or conjugate diene and acrylonitrile as constituent unit and into which are introduced ionic groups. This cleansing processing agent is contacted with a material for processing containing harmful substances contained in the material for processing for removing the harmful substances. The waste plastics can be used effectively, while a modified product obtained on chemical modification of the plastics is useful for suppressing environmental destruction caused by leakage of the harmful substances contained in the waste materials.