To provide an agent for decreasing harmful substances containing a cation adsorbent which has the ability of decreasing hexavalent chromium and the performance of removing harmful substances.

The agent for decreasing harmful substances contains slag produced from a desulfurization process and a cation adsorbent. An immediate or slow effect of decreasing hexavalent chromium can be obtained depending on the crushing conditions of the slag produced from the desulfurization process. The decreasing agent of harmful substances by mixing with mild soil, black sludge, sludge, construction waste soil, and industrial waste such as refuse incineration ash, sludge incineration ash, collected dust of the ash and fused slag decreases cadmium, lead and hexavalent chromium and various harmful substances. Trivalent chromium which is easily converted to the hexavalent chromium under oxidizing conditions can be also decreased.