To provide a wastewater treatment agent and a wastewater treatment method capable of treating wastewater at high precipitation rate and filtration rate and lowering the water content of sludge after dewatering and the turbidity of the filtered water.

This wastewater treatment agent contains sulfonated styrene- based polymer and/or its salts as an effectual component. The sulfonated styrene-based polymer and/or its salts has viscosity of 10-20,000 mPa.S at 25°C in the case the polymer and/or its neutral salts are added within 0.01-5 N to an aqueous solution of 1 wt.% concentration. Various wastewater is mixed with such wastewater treatment agent to be purified. Sulfonated styrene-based polymer and/or its salts having such property has excellently stable flocculation capability.