A method and apparatus for the treatment of an effluent containing an organic chlorine compound in the presence of a porous iron type metallic article and hydrogen peroxide thereby effecting oxidizing decomposition of the organic chlorine compound, operated by a procedure which comprises causing the organic chlorine compound having hydrogen peroxide incorporated preparatorily therein to be circulated through a closed vessel having the porous iron type metallic article disposed therein thereby effecting oxidizing decomposition of the organic chlorine compound and then simultaneously circulating and aerating the effluent containing the organic chlorine compound under the condition of keeping the upper part of the closed vessel in an opened state thereby effecting oxidizing decomposition of the organic chlorine compound or causing the gaseous phase stagnating in the upper part of the vessel to be introduced into the effluent.