Abstract: The treating material for polychlorobiphenyl-containing oil comprises calcium oxide provided with a hydrophobic property by deposition of an oil and capable of taking place water absorbing exothermic reaction upon decomposition of the oil, sepiolite, vermiculite or zeolite as a porous material having an ion exchanging capability, capable of adsorbing the polychlorobiphenyl-containing oil and the hydrophobic calcium oxide and decomposing the oil, and potassium carbonate, sodium carbonate and neutral detergent as an oil decomposing agent for decomposing the oil deposited on calcium oxide. By mixing the treating material with polychlorobiphenyl-containing oil, the oil component is decomposed into a powder at a low temperature and chlorine is liberated from polychlorobiphenyl by the ion exchanging capability and changed to a nontoxic chlorine compound such as calcium chloride or magnesium chloride.