To provide a decomposition treatment method for a halogenated organic compound, making alkali slurry hard to flocculate in mixing alkali slurry obtained by adding alkaline material to high boiling point hydrocarbon oil and grinding and dispersing the same with a halogenated organic compound, and heating the same to perform dehalogenation.

In this decomposition treatment method for a halogenated organic compound, halogen in a halogenated organic compound is heated in the presence of alkaline material in the state of being dissolved in high boiling point hydrocarbon oil. Dehalogenation is performed using alkali slurry obtained by adding alkaline material to high boiling point hydrocarbon oil, and grinding and dispersing the same. In that case, amphipathic compound such as stearic acid, which is oil soluble, and whose hydrophilic group is non-cation, is added as a flocculation preventing agent for alkaline material.